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                                                                   EXHIBIT 10.15

                           STOCK RESTRICTION AGREEMENT

     This STOCK RESTRICTION AGREEMENT (this "AGREEMENT"), dated as of
____________, is made by and between TolerRx, Inc., a Delaware corporation (the
"CORPORATION") and _____________________ (the "STOCKHOLDER").

     WHEREAS, it is deemed to be in the best interests of the Corporation and
the Stockholder that provision be made for the continuity and stability of the
business and policies of the Corporation and, to that end, the Corporation and
the Stockholder hereby set forth their agreement with respect to the shares of
Stock (as hereinafter defined) owned by the Stockholder.

     NOW, THEREFORE, in consideration of the mutual covenants and agreements
herein contained, the parties hereto, intending to be legally bound, hereby
agree as follows:

     SECTION 1. DEFINITIONS. As used herein, the following terms shall have the
following respective meanings:

          "ACT" shall mean the Securities Act of 1933, as amended.

          "CAUSE" shall mean, with respect to the Stockholder, the Stockholder's
(i) material misconduct with respect to the business and affairs of the
Corporation, (ii) material neglect of duties or material failure to act which
can reasonably be expected to adversely effect the business and affairs of the
Corporation, (iii) material breach of any of the terms hereof or of any
agreement between the Corporation and the Stockholder, including, but not
limited to terms relating to non-disclosure, non-competition and invention
assignment or (iv) an act by the Stockholder involving moral turpitude or
constituting fraud, a felony or other criminal act (other than a misdemeanor
traffic violation) or, with respect to the abuse of a controlled substance which
materially impairs the performance of the Stockholder's duties with respect to
the Corporation.

          "COMMON STOCK" shall mean the common stock, $0.001 par value per
share, of the Corporation.

          "COST PER SHARE" shall mean $        per share, subject to adjustment
to reflect any stock split, stock dividend, reverse stock split, stock
combination, reclassification, recapitalization, or other similar change
involving or affecting such the Corporation's Common Stock.

          "DESIGNATED PUBLIC OFFERING" shall mean a firm commitment underwritten
public offering of Common Stock of the Corporation registered under the Act,
pursuant to which (i) Common Stock is offered to the public at a price of at
least $2.00 (as set forth in the Certificate of Incorporation of the
Corporation, as amended, subject to adjustment to reflect stock splits, stock
dividends, stock combinations, recapitalizations and like occurrences, and (ii)
the net proceeds to the Corporation are at least $30,000,000.

          "NON-VESTED STOCK" shall mean all issued and outstanding Stock held by
the Stockholder which does not constitute Vested Stock.

          "SELL" "SALE" OR "SOLD", as to any Stock, shall mean to sell, or in
any other way, directly or indirectly, transfer, assign, distribute, encumber or
otherwise dispose of, either voluntarily or involuntarily; PROVIDED, HOWEVER,
that the terms "Sell" "Sale" or "Sold" shall not include the transfer, by gift
or otherwise, of any Stock by the Stockholder to any or all members of a class
of persons consisting

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of the Stockholder, his or her spouse or equivalent domestic partner and/or
descendants, or to a trust of which all of the beneficiaries are members of such
class (each, a "RELATED TRANSFEREE"); provided, that any such transfer to a
Related Transferee shall be permitted only on, and subject to, the express
conditions that:

               (i)    such Related Transferee shall be deemed to be a
Stockholder hereunder and shall hold the Stock subject to the provisions of this
Agreement; and

               (ii)   such Related Transferee executes all documents necessary
or desirable, in the reasonable judgment of the Corporation, to become a party
to, and be bound by the terms of, this Agreement, including, without limitation,
the Corporation's right to repurchase the Stock in the event of Termination of
Employment of the Stockholder set forth in Section 4 hereof, except that the
consideration to be paid with respect to such Stock on any such repurchase shall
be paid directly to such Related Transferee.

          "STOCK" shall mean (i)       shares of Common Stock of the Corporation
issued to the Stockholder as of the date hereof (ii) any additional shares of
capital stock of the Corporation into which such shares may be converted or for
which they may be exchanged or exercised; provided, however, that "Stock" shall
not include, for purposes of this Agreement, shares of Common Stock purchased by
the Stockholder through open market transactions using a registered
broker-dealer after the Corporation's initial underwritten public offering.

          "TERMINATION OF EMPLOYMENT" shall mean the termination for any reason
whatsoever of the employer-employee, corporation-director,
corporation-contractor or corporation-consultant relationship between the
Corporation and the Stockholder, including, but not limited to, termination by
resignation, discharge (with or without Cause), lapse of agreement, retirement,
death or disability.

          "VESTED STOCK" shall mean, with respect to any shares of Stock held by
the Stockholder, such number of shares of Stock which shall have become "vested"
as of the relevant date of determination pursuant to the following schedule:

<Table>
     <S>                                                                <C>
     Prior to the first anniversary of the Vesting Start Date           %
     On the first anniversary of the Vesting Start Date                 %
     On the second anniversary of the Vesting Start Date                %
     On the third anniversary of the Vesting Start Date                 %
     On the fourth anniversary of the Vesting Start Date                %
</Table>

          "VESTING START DATE" ____________ ____, ______

     SECTION 2. LIMITATIONS ON SALES OF STOCK BY THE STOCKHOLDER. The
Stockholder hereby agrees that she, he or it shall not at any time during the
term of this Agreement Sell any Stock, except for Sales of Vested Stock in
accordance with Section 3 hereof and except in connection with any repurchase by
the Corporation in accordance with Section 4 hereof.

     SECTION 3. PROCEDURES ON SALE OF VESTED STOCK TO THIRD PARTIES BY
STOCKHOLDER. Except as otherwise expressly provided herein, the Stockholder
hereby agrees that she, he or it shall not Sell any Vested Stock, except in
accordance with the following procedures:

          (a)  The Stockholder shall first deliver to the Corporation a written
notice (the "SECTION 3 OFFER NOTICE"), which Section 3 Offer Notice shall (i)
specifically identify the party or parties to whom or which the Stockholder
proposes to Sell Vested Stock (such party or parties hereinafter

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referred to as the "IDENTIFIED Parties"), pursuant to a BONA FIDE written offer
from such Identified Parties ("THIRD PARTY OFFER"), (ii) include a copy of the
Third Party Offer, and (iii) be irrevocable for a period of 60 days after
delivery thereof, offering to the Corporation, all of the Vested Stock proposed
to be Sold by the Stockholder to such Identified Parties at the purchase price
and on the terms and conditions of payment specified in the Third Party Offer.
The Corporation and/or its designee shall have the right and option, at their
sole discretion, for a period of 60 days after receipt of the Section 3 Offer
Notice, to accept any or all of the Vested Stock offered at the purchase price
and upon the terms and conditions of payment stated in the Section 3 Offer
Notice. Such acceptance will be made by delivery of a written notice to the
Stockholder within said 60-day period.

          (b)  Sales of Vested Stock under the terms of Section 3(a) above shall
be made at the offices of the Corporation on a mutually satisfactory business
day within 30 days after the expiration of the aforesaid period. Delivery of
certificates or other instruments evidencing such Vested Stock duly endorsed for
transfer shall be made on such date against payment of the purchase price
therefor.

          (c)  If effective acceptance shall not be received pursuant to
Section 3(a) above with respect to all Vested Stock offered for Sale pursuant to
the Section 3 Offer Notice, then the Stockholder may Sell to the Identified
Parties all, but not less than all, of the Vested Stock so offered for Sale and
not so accepted by the Corporation and/or its designee at a price not less than
the price, and on terms not more favorable to the purchaser thereof than the
terms, stated in the Section 3 Offer Notice at any time within 90 days after the
expiration of the 60-day period required by Section 3(a) above. In the event
that the Vested Stock is not Sold by the Stockholder during such 90-day period,
the right of the Stockholder to Sell such Vested Stock shall expire and the
obligations of this Section 3 shall be reinstated; PROVIDED, HOWEVER, that in
the event that the Stockholder determines, at any time during such 90-day
period, that the Sale of all or any part of the remaining Vested Stock on the
terms set forth in the Section 3 Offer Notice is impractical, the Stockholder
can terminate the offer and reinstate the procedure provided in this Section 3
without waiting for the expiration of such 90-day period.

          (d)  Anything contained herein to the contrary notwithstanding, any
purchaser of Vested Stock pursuant to this Section 3 who is not a signatory to
an agreement with the Corporation that is substantially similar to this
Agreement shall agree in writing in advance with the parties hereto to be bound
by and comply with all applicable provisions of this Agreement and shall be
deemed to be a Stockholder for all purposes of this Agreement.

          (e)  Anything contained herein to the contrary notwithstanding, Vested
Stock held by the Stockholder shall no longer be subject to this Section 3 upon
and after a Designated Public Offering.

     SECTION 4. RIGHTS TO REPURCHASE STOCK. The Stockholder hereby grants to the
Corporation certain repurchase rights with respect to the Stock as set forth in
this Section 4 and hereby agrees that she, he or it shall not Sell any Stock
that remains subject to such repurchase rights of the Corporation.

          (a)  In the event of Termination of Employment of the Stockholder, the
Corporation and/or its designee shall have the right and option, for a period of
60 days, to purchase from the Stockholder, and the Stockholder shall Sell or
cause to be Sold to the Corporation and/or its designee, as the case may be,
upon the exercise of such right, such number of shares of Non-Vested Stock and
Vested Stock (but only in the event of Termination of Employment for Cause)
owned by the Stockholder on the date of such Termination of Employment as is
specified by the Corporation and/or its designee, as the case may be, at a per
share price equal to the Cost Per Share. In the event of Termination of
Employment other than for Cause of the Stockholder, the Stockholder shall have
full right and title to all then Vested Stock and such Vested Stock shall not be
subject to the repurchase rights set forth in this Section 4(a). However, in
such event, the Stockholder shall execute an irrevocable voting proxy, in form
satisfactory to

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the Corporation, for the benefit of the Chief Executive Officer of the
Corporation for all of such Vested Stock. The repurchase rights provided in this
Section 4(a) shall be exercised by the Corporation or its designee, as the case
may be, by delivery to the Stockholder during the applicable aforesaid 60-day
period of a written notice of election to purchase such Stock.

          (b)  The number of shares of Stock subject to repurchase, at the time
of any stock dividend or other distribution made on or in respect of the shares
or any subdivision, combination, redemption or reclassification of the
outstanding capital stock of the Corporation or received in exchange for the
shares or any part thereof, shall be adjusted to give effect to such stock
dividend, other distribution, subdivision, combination, redemption or
reclassification.

          (c)  Sales of Stock effected under the terms of Section 4(a) hereof
shall be made at the offices of the Corporation on a mutually acceptable
business day within 15 days after the expiration of the period referred to in
Section 4(a). Delivery of certificates or other instruments evidencing such
Stock duly endorsed for transfer shall be made on such date against payment of
the purchase price therefor.

          (d)  Anything contained herein to the contrary notwithstanding, Vested
Stock held by the Stockholder shall no longer be subject to this Section 4 upon
and after a Designated Public Offering.

          (e)  In the event of a sale of the Corporation (whether by merger,
sale of all or substantially all of the assets or sale or other disposition of
greater than 50% of the voting capital stock (in a single transaction or series
of related transactions, occurring over a limited period of time, including a
recapitalization), and if the Stockholder experiences a Termination of
Employment other than for Cause within six months after the close of such sale,
then 100% of the Stock held by the Stockholder shall become Vested Stock.

     SECTION 5. ACKNOWLEDGEMENTS AND COVENANTS; LEGENDS ON STOCK CERTIFICATES.

          (a)  In addition to complying with all other provisions hereof, the
Stockholder further acknowledges and agrees that:

               (i)    The Stock has not been registered under the Act, or any
     applicable state securities laws by reason of exemptions from the
     registration requirements of the Act and such laws, and the Stock (or any
     part thereof) may not be sold, transferred or otherwise disposed of in the
     absence of an effective registration statement for the Stock under the Act
     or unless an exemption from such registration is available.

               (ii)   The Stockholder will not attempt to sell, transfer or
     otherwise dispose of all or any portion of the Stock in the absence of an
     effective registration statement or an opinion of reputable securities
     counsel satisfactory in form and substance to the Corporation and its
     counsel that such proposed sale, transfer or other disposition would not be
     in violation of the Act and applicable state securities laws.

               (iii)  Appropriate restrictive endorsement(s) will be placed upon
     the certificates evidencing the Stock to reflect the foregoing and that the
     Corporation will give appropriate stop transfer instructions to the
     person(s) in charge of the transfer of its securities.

          (b)  Certificates representing the Stock issued to the Stockholder
will bear legends in substantially the following forms:

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     The securities represented by this certificate have been acquired for
     investment and have not been registered under the Securities Act of
     1933, as amended, or any state securities laws. The securities may not
     be pledged, hypothecated, sold or transferred in the absence of an
     effective registration statement for the securities under the
     Securities Act of 1933 and applicable state securities laws or an
     opinion of counsel satisfactory to TolerRx, Inc. that such pledge,
     hypothecation, sale or transfer is exempt therefrom under any such Act
     and applicable state securities laws.

     The transfer of this certificate and the shares represented by this
     certificate are subject to the terms and conditions specified in the
     TolerRx, Inc. 2000 Equity Incentive Plan, as amended, and the Stock
     Restriction Agreement, dated as of __________ between TolerRx, Inc.
     and the holder of record of this certificate, and no transfer of this
     certificate or the shares represented by this certificate shall be
     valid or effective until such terms and conditions have been
     fulfilled. Copies of such Plan and Agreement may be obtained at no
     cost by written request made by the holder of record of this
     certificate to the Secretary of TolerRx, Inc.

     SECTION 6. UNDERWRITER'S LOCK-UP. Notwithstanding anything contained herein
to the contrary, the Stockholder agrees not to Sell any Stock for such period of
time after the consummation of an underwritten public offering of capital stock
of the Corporation, not to exceed 180 days after the closing of such
underwritten public offering, as may be requested by the managing underwriter of
such underwritten public offering and as may be agreed to by the Corporation in
order to effectuate such offering.

     SECTION 7. ADDITIONAL SHARES OF STOCK; ETC. In the event additional shares
of Stock are issued by the Corporation to the Stockholder at any time during the
term of this Agreement, either directly or upon the exercise, conversion or
exchange of securities of the Corporation exercisable for or convertible or
exchangeable into shares of Stock, such additional shares of Stock shall, as a
condition to such issuance, become subject to the terms and provisions of this
Agreement.

     SECTION 8. DURATION OF AGREEMENT. Except as otherwise stated herein, the
rights and obligations of the Stockholder under this Agreement shall terminate
upon the earlier to occur of (a) the Sale of all Stock owned by the Stockholder
in accordance with the terms hereof and (b) on the fifteenth (15th) anniversary
of the date of this Agreement, PROVIDED, in each case, that the obligations of
the Stockholder set forth in Sections 5 - 7 of this Agreement shall survive.

     SECTION 9. SEVERABILITY; GOVERNING LAW. If any provisions of this Agreement
shall be determined to be illegal and unenforceable by any court of law, the
remaining provisions shall be severable and enforceable in accordance with their
terms. This Agreement shall be governed by, and construed in accordance with,
the laws of the State of Delaware applicable to contracts made and to be
performed wholly therein.

     SECTION 10. SUCCESSORS AND ASSIGNS. This Agreement shall bind and inure to
the benefit of the parties and their respective successors and permitted
assigns, legal representatives and heirs.

     SECTION 11. NOTICES. All notices, requests, consents and other
communications hereunder to any party shall be deemed to be sufficient if
contained in a written instrument delivered in person or by telecopy of sent by
nationally-recognized overnight courier or first class registered or certified
mail, return receipt requested, postage prepaid, addressed to such party at the
address set forth below or at such other address as may hereafter be designated
in writing by such party to the other parties:

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          (a)    if to the Corporation, to:

                 TolerRx, Inc.
                 300 Technology Square
                 Cambridge, Massachusetts 02139
                 Attention:  Douglas J. Ringler
                 Telecopier:  (617) 354-8300

                 with a copy to:

                 Bingham McCutchen LLP
                 150 Federal Street
                 Boston, Massachusetts 02110
                 Attention:  Julio E. Vega, Esquire
                 Telecopier:  (617) 951-8000

          (b)    if to the Stockholder, to the address set forth below the
                 Stockholder's name on the signature page hereto.

All such notices, requests, consents and other communications shall be deemed to
have been delivered (i) in the case of personal delivery or delivery by
telecopy, on the date of such delivery, (ii) in the case of dispatch by
nationally-recognized overnight courier, on the next business day following such
dispatch and (iii) in the case of mailing, on the third business day after the
posting thereof.

     SECTION 12. MODIFICATION. Except as otherwise provided herein, neither this
Agreement nor any provisions hereof can be modified, waived, changed, discharged
or terminated except by an instrument in writing signed by the party against
whom the enforcement of any modification, waiver, change, discharge or
termination is sought.

     SECTION 13. HEADINGS. The headings of the sections of this Agreement have
been inserted for convenience of reference only and shall not be deemed to be a
part of this Agreement.

     SECTION 14. NOUNS AND PRONOUNS. Whenever the context may require, any
pronouns used herein shall include the corresponding masculine, feminine or
neuter forms, and the singular form of names and pronouns shall include the
plural and vice-versa.

     SECTION 15. ENTIRE AGREEMENT. This Agreement and the other writings
referred to herein or delivered pursuant hereto contain the entire agreement
among the parties hereto with respect to the subject matter hereof and supersede
all prior and contemporaneous written or oral agreements and understandings with
respect thereto.

     SECTION 16. COUNTERPARTS. This Agreement may be executed in any number of
counterparts, and each such counterpart hereof shall be deemed to be an original
instrument, but all such counterparts together shall constitute but one
agreement.

                            [SIGNATURE PAGE FOLLOWS]

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     IN WITNESS WHEREOF, the parties hereto have executed this Stock Restriction
Agreement as of the date first above written.

                                        CORPORATION:

                                        TOLERRX, INC.

                                        By:
                                            --------------------------

                                        STOCKHOLDER:

                                        ------------------------------
                                        Name:

                                        Address:
                                                 ---------------------

                                        ------------------------------

                                        ------------------------------

                                        Facsimile:
                                                   -------------------<Page>

                                                                   Exhibit 10.16

THIS WARRANT HAS NOT BEEN REGISTERED UNDER THE SECURITIES ACT OF 1933, AS
AMENDED, OR APPLICABLE STATE SECURITIES LAWS. THIS WARRANT MAY NOT BE SOLD,
OFFERED FOR SALE, MORTGAGED, PLEDGED HYPOTHECATED OR OTHERWISE TRANSFERRED IN
THE ABSENCE OF AN EFFECTIVE REGISTRATION STATEMENT AS TO THIS WARRANT UNDER SAID
ACT AND APPLICABLE STATE SECURITIES LAWS OR AN OPINION OF COUNSEL ACCEPTABLE TO
TOLERRX, INC. THAT SUCH REGISTRATION IS NOT REQUIRED.

                                                               November 20, 2001

                                  TOLERRX, INC.

                          COMMON STOCK PURCHASE WARRANT

         This Warrant (the "WARRANT") entitles MASSACHUSETTS INSTITUTE OF
TECHNOLOGY (the "HOLDER"), for value received, to purchase from TOLERRX, INC., a
Delaware corporation (the "COMPANY"), at any time, subject to the restrictions
set forth in Section 2 hereof, during the period starting from 5:00 a.m. on the
Initial Exercise Date (as defined in Section 1 below) to 5:00 p.m., Boston time,
on the Expiration Date (as defined in Section 1 below), at which time this
Warrant shall expire and become void, up to ONE HUNDRED THOUSAND (100,000)
shares of the Company's common stock, $0.001 par value per share, subject to
adjustment as set forth herein (the "WARRANT SHARES"), for a price per share of
$1.00, subject to adjustment as set forth herein (the "EXERCISE PRICE"). This
Warrant also is subject to the following terms and conditions:

         1.       DEFINITIONS. As used in this Warrant, the following terms
shall have the respective meanings set forth below or elsewhere in this Warrant
as referred to below:

         "COMMON STOCK" shall mean common stock, $0.001 par value per share, of
the Company.

         "COMPANY" shall have the meaning set forth in the preamble of this
Warrant.

         "EXERCISE PRICE" shall have the meaning set forth in the preamble of
this Warrant.

         "EXPIRATION DATE" shall mean the earliest to occur of (i) the one year
anniversary of the date of termination of the lease agreement (the "LEASE"),
dated November 20, 2001, between the Company and the Holder, for space at 300
Technology Square, Cambridge, Massachusetts, or (ii) the closing of a
consolidation or merger of the Company with another corporation (other than a
merger in which the Company is the surviving corporation) or the sale of all or
substantially all of the Company's assets ("SALE OF THE COMPANY"), provided;

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                                      -2-

however, that the Holder receives written notice of such transaction at least 20
days prior to the consummation thereof.

         "HOLDER" shall have the meaning set forth in the preamble of this
Warrant.

         "INITIAL EXERCISE DATE" shall mean the date hereof.

         "SECURITIES ACT" shall mean the Securities Act of 1933, as amended.

         "WARRANT SHARES" shall have the meaning set forth in the preamble to
this Warrant.

         2.       EXERCISE OF WARRANT. Subject to the terms and conditions
hereof, this Warrant may be exercised in whole or in part at any time from and
after the Initial Exercise Date and before the Expiration Date, PROVIDED that
this Warrant shall become exercisable, cumulatively, in a series of five (5)
installments, with the first installment of 20% to become exercisable on the
date hereof and the remaining four installments of 20% each to become
exercisable, successively, on the first four anniversaries of the Rent
Commencement Date (as defined in the Lease). Exercise shall be by presentation
and surrender to the Company at its principal office of this Warrant and the
subscription form annexed hereto, executed by the Holder, together with payment
to the Company in accordance with Section 3.1 or Section 3.2 hereof in an amount
equal to the product of the Exercise Price multiplied by the number of Warrant
Shares being purchased upon such exercise. It shall be a condition precedent to
the exercise of this Warrant, in whole or in part, that the Holder shall deliver
to the Company a certificate certifying that the representations set forth in
Section 9 hereof are true and correct as of the date of such exercise. If this
Warrant is exercised in part only, the Company shall, as soon as practicable
after presentation of this Warrant upon such exercise, execute and deliver a new
Warrant, dated the date hereof, evidencing the right of the Holder to purchase
the balance of the Warrant Shares purchasable hereunder upon the same terms and
conditions herein set forth. Upon and as of receipt by the Company of such
properly completed and duly executed purchase form accompanied by payment as
herein provided, the Holder shall be deemed to be the Holder of record of the
Warrant Shares issuable upon such exercise, notwithstanding that the stock
transfer books of the Company shall then be closed or that certificates
representing such Warrant Shares shall not then actually be delivered to the
Holder.

         3.       PAYMENT OF EXERCISE PRICE.

                  3.1.     CASH PAYMENT. The Exercise Price for the Warrant
Shares being purchased may be paid in cash or by bank check.

                  3.2.     NET ISSUE ELECTION. The Holder may elect to receive,
without the payment by the Holder of any additional consideration, shares equal
to the value of this Warrant or any portion hereof by the surrender of this
Warrant or such portion to the Company, with the net issue election notice
annexed hereto duly executed, at the office of

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                                      -3-

the Company. Thereupon, the Company shall issue to the Holder such number of
fully paid and nonassessable shares of Common Stock as is computed using the
following formula:

                                   X = Y (A-B)
                                       -------
                                          A

where:

                  X =      the number of shares to be issued to the Holder
                           pursuant to this Section 3.2.

                  Y =      the number of shares of Common Stock covered by this
                           Warrant in respect of which the net issue election is
                           made.

                  A =      the fair market value of one share of Common Stock,
                           as determined in good faith by the Board, as at the
                           time the net issue election is made.

                  B =      the Exercise Price in effect under this Warrant at
                           the time the net issue election is made.

The Board shall promptly respond in writing to an inquiry by the Holder as to
the fair market value of one share of Common Stock.

         4.       ADJUSTMENT OF EXERCISE PRICE. The Exercise Price shall be
subject to adjustment from time to time upon the happening of certain events as
follows:

                  4.1.     SUBDIVISION OR COMBINATION OF STOCK. If at any time
or from time to time after the date hereof the Company shall subdivide its
outstanding shares of Common Stock, the Exercise Price in effect immediately
prior to such subdivision shall be reduced proportionately and the number of
Warrant Shares (calculated to the nearest whole share) shall be increased
proportionately, and conversely, in the event the outstanding shares of Common
Stock shall be combined into a smaller number of shares, the Exercise Price in
effect immediately prior to such combination shall be increased proportionately
and the number of Warrant Shares (calculated to the nearest whole share) shall
be decreased proportionately.

                  4.2      ADJUSTMENT FOR STOCK DIVIDENDS. If at any time after
the date hereof the Company shall declare a dividend or make any other
distribution upon any class or series of stock of the Company payable in shares
of Common Stock or securities convertible into shares of Common Stock, the
Exercise Price and the number of shares to be obtained upon exercise of this
Warrant shall be adjusted proportionately to reflect the issuance of any shares
of Common Stock or convertible securities, as the case may be, issuable in
payment of such dividend or distribution.

                  4.3      REORGANIZATION OR RECLASSIFICATION. In the event of
any reorganization, recapitalization or reclassification of the capital stock of
the Company or any

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                                      -4-

transaction involving the transfer of a majority of the voting power over the
capital stock of the Company effected in a manner such that holders of Common
Stock shall be entitled to receive stock, securities, or other assets or
property, in each case, at any time after the date hereof, then, as a condition
of such reorganization or reclassification, lawful and adequate provision shall
be made whereby the Holder hereof shall have the right to purchase and receive
(in lieu of the shares of the Common Stock of the Company immediately
theretofore purchasable and receivable upon the exercise of the rights
represented hereby) such shares of stock, securities or other assets or property
as may be issued or payable with respect to or in exchange for a number of
outstanding shares of such Common Stock equal to the number of shares of such
Common Stock immediately theretofore purchasable and receivable upon the
exercise of the rights represented hereby. In any such reorganization or
reclassification transaction, including successive events of such nature,
appropriate provision shall be made with respect to the rights and interests of
the Holder such that the provisions hereof (including, without limitation,
provisions for adjustments of the Exercise Price and of the number of shares
purchasable and receivable upon the exercise of this Warrant) thereafter shall
be applicable, as nearly practicable, in relation to any shares of stock,
securities or assets thereafter deliverable upon the exercise hereof.

                  4.4      CERTIFICATE AS TO ADJUSTMENTS. Upon the occurrence of
each adjustment or readjustment of the Exercise Price and the number of shares
to be obtained upon exercise of this Warrant pursuant to this Section 4, the
Warrant shall, without any action on the part of the holder thereof, be adjusted
in accordance with this Section 4, and the Company promptly shall compute such
adjustment or readjustment in accordance with the terms hereof and prepare and
furnish to the Holder a certificate setting forth such adjustment or
readjustment, showing in detail the facts upon which such adjustment or
readjustment is based.

         5.       NOTICES OF RECORD DATE; EXERCISE. Upon (a) any establishment
by the Company of a record date of the holders of any class of securities for
the purpose of determining the holders thereof who are entitled to receive any
dividend or other distribution, or right or option to acquire securities of the
Company, or any other right, (b) any capital reorganization, reclassification,
recapitalization, or Sale of the Company, or any voluntary or involuntary
dissolution, liquidation or winding up of the Company, or (c) the closing of an
initial public offering of shares of the Company's common Stock (the "IPO"), the
Company shall mail to the Holder at least 15 days, or such longer period as may
be required by law, prior to the record date specified therein, a notice
specifying (i) the date established as the record date for the purpose of such
dividend, distribution, option or right and a description of such dividend,
distribution, option or right, (ii) the date on which any such reorganization,
recapitalization, reclassification, Sale of the Company or dissolution,
liquidation or winding up is expected to become effective, (iii) the date, if
any, fixed as to when the holders of record of Common Stock shall be entitled to
exchange their shares of Common Stock for securities or other property
deliverable upon such reorganization, recapitalization, reclassification, Sale
of the Company, dissolution, liquidation or winding up and (iv) the expected
date of the IPO. The Holder shall, within 10 days of receipt of such

<Page>

                                      -5-

notice, exercise this Warrant in accordance with Section 2 hereof, otherwise
this Warrant shall automatically terminate.

         6.       NO DILUTION OR IMPAIRMENT. The Company will not, by amendment
of its Certificate of Incorporation or By-Laws or through any reorganization,
transfer of assets, consolidation, merger, dissolution, issue or sale of
securities or any other voluntary action, avoid or seek to avoid the observance
or performance of any of the terms of this Warrant, but will at all times in
good faith assist in the carrying out of all such terms and in the taking of all
action as may be necessary or appropriate in order to protect the rights of the
Holder against dilution, or other impairment.

         7.       FRACTIONAL SHARES. The Company shall not issue any fractional
shares nor scrip representing fractional shares upon exercise of any portion of
this Warrant.

         8.       REPRESENTATIONS, WARRANTIES AND COVENANTS. This Warrant is
issued and delivered by the Company and accepted by each Holder on the basis of
the following representations, warranties and covenants made by the Company:

                  8.1.     AUTHORITY. The Company has all necessary authority to
issue, execute and deliver this Warrant and to perform its obligations
hereunder. This Warrant has been duly authorized, issued, executed and delivered
by the Company and is the valid and binding obligation of the Company,
enforceable in accordance with its terms.

                  8.2.     RESERVATION OF WARRANT SHARES. The Warrant Shares
issuable upon the exercise of this Warrant have been (and any securities
issuable or deliverable upon conversion of such Warrant Shares, upon issuance or
delivery, will be) duly authorized and reserved for issuance by the Company and,
when issued in accordance with the terms hereof, will be validly issued, fully
paid and nonassessable.

         9.       INVESTMENT REPRESENTATIONS.

                  9.1.     PURCHASE FOR INVESTMENT. The Holder represents and
warrants that it is acquiring the Warrant, and upon exercise will hold the
Warrant Shares, solely for its account for investment and not with a view to or
for sale or distribution of said Warrant or Warrant Shares in violation of the
Securities Act. The Holder also represents that the entire legal and beneficial
interests of the Warrant and Warrant Shares the Holder is acquiring is being
acquired for, and will be held for, its account only.

                  9.2.     SECURITIES NOT REGISTERED. The Holder understands
that the Warrant has not been registered under the Securities Act on the basis
that no distribution or public offering of the stock of the Company is to be
effected. The Holder realizes that the basis for the exemption may not be
present if, notwithstanding its representations, it has in mind merely acquiring
the securities for a fixed or determinable period in the future, or for a market
rise, or for sale if the market does not rise. The Holder has no such intention.

<Page>

                                      -6-

                  9.3.     SECURITIES TO BE HELD INDEFINITELY. The Holder
recognizes that the Warrant and Warrant Shares being acquired by it must be held
indefinitely unless they are subsequently registered under the Securities Act or
an exemption from such registration is available. The Holder recognizes that the
Company has no obligation to register the Warrant or to comply with any
exemption from such registration.

                  9.4.     RULE 144. The Holder is aware that neither the
Warrant nor Warrant Shares may be sold pursuant to Rule 144 adopted under the
Securities Act unless certain conditions are met and until the Holder has held
the Warrant Shares for at least one year. Among the conditions for use of Rule
144 is the availability to the public of current information about the Company.

         10.      TRANSFER, EXCHANGE, ASSIGNMENT OR LOSS OF WARRANT.

                  10.1.    RESTRICTIONS ON TRANSFER. This Warrant may not be
transferred, in whole or in part, without the prior written consent of the
Company. The Warrant Shares received upon exercise of this Warrant may not be
transferred, in whole or in part, without the prior written consent of the
Company prior to the Company's IPO. This Warrant and the Warrant Shares received
upon exercise of this Warrant shall be subject to restrictions on
transferability until registered under the Securities Act, unless an exemption
from registration is available.

                  10.2.    EFFECT OF VIOLATION OF TRANSFER RESTRICTIONS;
PREVENTIVE MEASURES. Any offer, sale, assignment, transfer, endorsement, pledge,
mortgage, hypothecation, or other conveyance or disposition of all or any
portion of this Warrant or any Warrant Shares issued from time to time upon
exercise of this Warrant, or of any interest in this Warrant or any of such
Warrant Shares, in violation of this Section 10 shall be null and void. The
Company may make a notation on its records or give instructions to any of its
transfer agents in order to implement the restrictions on transfer set forth in
this Section 10. The Company shall not incur any liability for any delay in
recognizing any transfer of this Warrant or of any Warrant Shares issued from
time to time upon exercise of this Warrant if the Company reasonably believes
that any such transfer may have been or would be in violation of the provisions
of the Securities Act, applicable blue sky laws or this Section 10.

                  10.3.    LEGENDS. Each certificate representing any Warrant
Shares issued upon exercise of this Warrant shall bear the following legend:

         "THE SHARES REPRESENTED BY THIS CERTIFICATE HAVE NOT BEEN
         REGISTERED UNDER THE SECURITIES ACT OF 1933, AS AMENDED,
         OR APPLICABLE STATE SECURITIES LAWS. THE SHARES MAY NOT BE
         SOLD, OFFERED FOR SALE, MORTGAGED, PLEDGED HYPOTHECATED OR
         OTHERWISE TRANSFERRED IN THE ABSENCE OF AN EFFECTIVE
         REGISTRATION STATEMENT AS TO THE SHARES UNDER SAID ACT AND
         APPLICABLE STATE SECURITIES LAWS OR AN OPINION OF COUNSEL
         ACCEPTABLE

<Page>

                                      -7-

         TO TOLERRX, INC. THAT SUCH REGISTRATION IS NOT REQUIRED."

                  10.4.    SURVIVAL. The obligations of the Holder (and/or of
any transferee of the Warrant or any Warrant Shares issued from time to time
upon exercise of this Warrant) under this Section 10 shall, with respect to any
Warrant Shares issued upon exercise of this Warrant, survive the exercise,
expiration or other termination, or transfer, of this Warrant indefinitely.

                  10.5.    PROCEDURE FOR TRANSFER. Any transfer permitted
hereunder shall be made by surrender of this Warrant to the Company at its
principal office or to the Transfer Agent at its offices with a duly executed
request to transfer the Warrant, which shall provide adequate information to
effect such transfer and shall be accompanied by funds sufficient to pay any
transfer taxes applicable. Upon satisfaction of all transfer conditions, the
Company or Transfer Agent shall, without charge, execute and deliver a new
Warrant in the name of the transferee named in such transfer request, and this
Warrant promptly shall be canceled.

                  10.6.    LOCK-UP AGREEMENT. Holder hereby agrees that, at the
written request of the Company or any managing underwriter of any underwritten
public offering of securities of the Company, such Holder shall not, without the
prior written consent of the Company or such managing underwriter, sell, assign,
transfer, endorse, pledge, mortgage, hypothecate, or otherwise convey or dispose
(whether by gift, operation of law or otherwise) of any shares of Common Stock
for such period of time, not to exceed 180 days after the effective date of the
registration statement relating to such underwritten public offering, as the
Company or such managing underwriter shall request, PROVIDED THAT persons who
hold in excess of 5% of the Common Stock of the Company, officers, directors and
key employees of the Company also enter into similar agreement.

                  10.7.    LOST, STOLEN OR DESTROYED WARRANT. Upon receipt by
the Company of evidence satisfactory to it of loss, theft, destruction or
mutilation of this Warrant and, in the case of loss, theft or destruction, of
reasonably satisfactory indemnification, or, in the case of mutilation, upon
surrender of this Warrant, the Company will execute and deliver, or instruct the
Transfer Agent to execute and deliver, a new Warrant of like tenor and date, and
any such lost, stolen or destroyed Warrant thereupon shall become void.

                  10.8.    WARRANT BINDING UPON ASSIGNEE OR SUCCESSOR. The terms
and conditions of this Warrant shall be binding upon any permitted assignee and
successor of the Holder. Any such successor or assignee shall be obligated to
and shall immediately execute an instrument which provides that such party is
bound under the terms of this Warrant. Any transfer, assignment or other
disposition without such execution by the proposed transferee, assignee or
successor shall be null and void.

         11.      ISSUE TAX. The issuance of certificates for shares of Common
Stock upon the exercise of this Warrant shall be made without charge to the
Holder of the Warrant for any issue tax (other than applicable income taxes) in
respect thereof; provided, however, that the

<Page>

                                      -8-

Company shall not be required to pay any tax that may be payable in respect of
any transfer involved in the issuance and delivery of any certificate in a name
other than that of the then Holder of the Warrant being exercised.

         12.      NO RIGHTS AS STOCKHOLDER. The Holder shall not be entitled to
vote or to receive dividends or to be deemed the holder of Common Stock that may
at any time be issuable upon exercise of this Warrant for any purpose
whatsoever, nor shall anything contained herein be construed to confer upon the
Holder any of the rights of a stockholder of the Company or any right to vote
for the election of directors or upon any matter submitted to stockholders at
any meeting thereof, or to give or withhold consent to any corporate action
(whether upon any recapitalization, issuance or reclassification of stock,
change of par value or change of stock to no par value, consolidation, merger or
conveyance or otherwise), or to receive notice of meetings (except to the extent
otherwise provided in this Warrant), or to receive dividends or subscription
rights, until the Holder shall have exercised the Warrant and shall have been
issued Warrant Shares in accordance with the provisions hereof.

         13.      NOTICES. All notices, demands, requests, certificates or other
communications under this Warrant shall be in writing and shall be telegraphed,
mailed by certified or registered mail with charges prepaid, hand delivered or
sent by facsimile transmission, by tested or otherwise authenticated telex or
cable or by commercial courier guaranteeing two business days' delivery:

         (a)      if to the Company, at 675 Massachusetts Avenue, Cambridge, MA
02139, Attention: President, or at such other address as the Company may have
furnished in writing to the Holder; and

         (b)      if to Holder, at Holder's address appearing in the books
maintained by the Company.

         14.      AMENDMENT. The terms of this Warrant may be amended, modified
or waived only with the written consent of the parties hereto.

         15.      GOVERNING LAW. This Warrant shall be governed by and construed
in accordance with the laws of the Commonwealth of Massachusetts, as such laws
are applied to contracts entered into and wholly to be performed within the
Commonwealth of Massachusetts and without giving effect to any principles of
conflicts or choice of law that would result in the application of the laws of
any other jurisdiction.

                            [SIGNATURE PAGE FOLLOWS.]

<Page>

                                      -9-

         IN WITNESS WHEREOF, the Company and the Holder have executed this
Warrant as of November 20, 2001.

COMPANY:                              TOLERRX, INC.

                                      By:  /s/ Douglas J. Ringler
                                         ---------------------------------------
                                         Douglas J. Ringler
                                         President and Chief Executive Officer

HOLDER:                               MASSACHUSETTS INSTITUTE
                                      OF TECHNOLOGY

                                      By: /s/ Stephen C. Marsh
                                         ---------------------------------------
                                      Name:  Stephen C. Marsh
                                      Title:  Managing Director, Real Estate

<Page>

                                      -10-

                                  SUBSCRIPTION

To:                                      Date:
   ----------------------------------         ----------------------------------

         The undersigned hereby subscribes for __________________ shares of
Common Stock covered by this Warrant. The certificate(s) for such shares shall
be issued in the name of the undersigned or as otherwise indicated below:

                                         ---------------------------------------
                                         Signature

                                         ---------------------------------------
                                         Name for Registration

                                         ---------------------------------------
                                         Mailing Address

<Page>

                            NET ISSUE ELECTION NOTICE

To:                                      Date:
   ----------------------------------         ----------------------------------

         The undersigned hereby elects under Section 3.2 to surrender the right
to purchase ________ shares of Common Stock pursuant to this Warrant. The
certificate(s) for the shares issuable upon such net issue election shall be
issued in the name of the undersigned or as otherwise indicated below.

                                         ---------------------------------------
                                         Signature

                                         ---------------------------------------
                                         Name for Registration

                                         ---------------------------------------
                                         Mailing Address

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