QuickLinks -- Click here to rapidly navigate through this document

Exhibit 10.3

NEITHER THIS SECURITY NOR THE SECURITIES INTO WHICH THIS SECURITY IS EXERCISABLE
HAVE BEEN REGISTERED WITH THE SECURITIES AND EXCHANGE COMMISSION OR THE
SECURITIES COMMISSION OF ANY STATE IN RELIANCE UPON AN EXEMPTION FROM
REGISTRATION UNDER THE SECURITIES ACT OF 1933, AS AMENDED (THE "SECURITIES
ACT"), AND, ACCORDINGLY, MAY NOT BE OFFERED OR SOLD EXCEPT PURSUANT TO AN
EFFECTIVE REGISTRATION STATEMENT UNDER THE SECURITIES ACT OR PURSUANT TO AN
AVAILABLE EXEMPTION FROM, OR IN A TRANSACTION NOT SUBJECT TO, THE REGISTRATION
REQUIREMENTS OF THE SECURITIES ACT AND IN ACCORDANCE WITH APPLICABLE STATE
SECURITIES LAWS AS EVIDENCED BY A LEGAL OPINION OF COUNSEL TO THE TRANSFEROR TO
SUCH EFFECT, THE SUBSTANCE OF WHICH SHALL BE REASONABLY ACCEPTABLE TO THE
COMPANY. THIS SECURITY AND THE SECURITIES ISSUABLE UPON EXERCISE OF THIS
SECURITY MAY BE PLEDGED IN CONNECTION WITH A BONA FIDE MARGIN ACCOUNT OR OTHER
LOAN SECURED BY SUCH SECURITIES.

COMMON STOCK PURCHASE WARRANT

To Purchase 444,444 Shares of Common Stock of

AVANT Immunotherapeutics, Inc.

        THIS COMMON STOCK PURCHASE WARRANT CERTIFIES that, for value received,
The Riverview Group, LLC (the "Holder"), is entitled, upon the terms and subject
to the limitations on exercise and the conditions hereinafter set forth, at any
time on or after July 1, 2003 (the "Initial Exercise Date") and on or prior to
the close of business on June 30, 2008 (the "Termination Date") but not
thereafter, to subscribe for and purchase from AVANT Immunotherapeutics, Inc., a
corporation incorporated in the State of Delaware (the "Company"), up to 444,444
shares (the "Warrant Shares") of Common Stock, par value $0.001 per share, of
the Company (the "Common Stock"). The purchase price of one share of Common
Stock (the "Exercise Price") under this Warrant shall be $3.00, subject to
adjustment hereunder. The Exercise Price and the number of Warrant Shares for
which the Warrant is exercisable shall be subject to adjustment as provided
herein. Capitalized terms used and not otherwise defined herein shall have the
meanings set forth in that certain Securities Purchase Agreement (the "Purchase
Agreement"), dated July 1, 2003, between the Company and the purchasers
signatory thereto.

        1.    Title to Warrant.    Prior to the Termination Date and subject to
compliance with applicable laws and Section 7 of this Warrant, this Warrant and
all rights hereunder are transferable, in whole or in part, at the office or
agency of the Company by the Holder in person or by duly authorized attorney,
upon surrender of this Warrant together with the Assignment Form annexed hereto
properly endorsed. The transferee shall sign an investment letter in form and
substance reasonably satisfactory to the Company.

        2.    Authorization of Shares.    The Company covenants that all Warrant
Shares which may be issued upon the exercise of the purchase rights represented
by this Warrant will, upon exercise of the purchase rights represented by this
Warrant, be duly authorized, validly issued, fully paid and nonassessable and
free from all taxes, liens and charges in respect of the issue thereof (other
than taxes in respect of any transfer occurring contemporaneously with such
issue).

        3.    Exercise of Warrant.    

        (a)   Exercise of the purchase rights represented by this Warrant may be
made at any time or times on or after the Initial Exercise Date and on or before
the Termination Date by the surrender of this Warrant and the Notice of Exercise
Form annexed hereto duly executed, at the office of the

1

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

Company (or such other office or agency of the Company as it may designate by
notice in writing to the registered Holder at the address of such Holder
appearing on the books of the Company) and upon payment of the Exercise Price of
the shares thereby purchased by wire transfer or cashier's check drawn on a
United States bank or by means of a cashless exercise pursuant to Section 3(d),
if applicable, the Holder shall be entitled to receive a certificate for the
number of Warrant Shares so purchased. Certificates for shares purchased
hereunder shall be delivered to the Holder within three (3) Trading Days after
the date on which this Warrant shall have been exercised as aforesaid. This
Warrant shall be deemed to have been exercised and such certificate or
certificates shall be deemed to have been issued, and the Holder or any other
person so designated to be named therein shall be deemed to have become a holder
of record of such shares for all purposes, as of the date the Warrant has been
exercised by payment to the Company of the Exercise Price and all taxes required
to be paid by the Holder, if any, pursuant to Section 5 prior to the issuance of
such shares, have been paid. If the Company fails to deliver to the Holder a
certificate or certificates representing the Warrant Shares pursuant to this
Section 3(a) by the close of business on the fifth (5th) Trading Day after the
date of exercise, then the Holder will have the right to rescind such exercise.
In addition to any other rights available to the Holder, if the Company fails to
deliver to the Holder a certificate or certificates representing the Warrant
Shares pursuant to an exercise by the close of business on the third Trading Day
after the date of exercise, and if after such fifth (5th) Trading Day the Holder
is required by its broker to purchase (in an open market transaction or
otherwise) shares of Common Stock to deliver in satisfaction of a sale by the
Holder of the Warrant Shares which the Holder anticipated receiving upon such
exercise (a "Buy-In"), then the Company shall (1) pay in cash to the Holder the
amount by which (x) the Holder's total purchase price (including brokerage
commissions, if any) for the shares of Common Stock so purchased exceeds (y) the
amount obtained by multiplying (A) the number of Warrant Shares that the Company
was required to deliver to the Holder in connection with the exercise at issue
times (B) the price at which the sell order giving rise to such purchase
obligation was executed, and (2) at the option of the Holder, either reinstate
the portion of the Warrant and equivalent number of Warrant Shares for which
such exercise was not honored or deliver to the Holder the number of shares of
Common Stock that would have been issued had the Company timely complied with
its exercise and delivery obligations hereunder. For example, if the Holder
purchases Common Stock having a total purchase price of $11,000 to cover a
Buy-In with respect to an attempted exercise of shares of Common Stock with an
aggregate sale price giving rise to such purchase obligation of $10,000, under
clause (1) of the immediately preceding sentence the Company shall be required
to pay the Holder $1,000. The Holder shall provide the Company written notice
indicating the amounts payable to the Holder in respect of the Buy-In, together
with applicable confirmations and other evidence reasonably requested by the
Company. Nothing herein shall limit a Holder's right to pursue any other
remedies available to it hereunder, at law or in equity including, without
limitation, a decree of specific performance and/or injunctive relief with
respect to the Company's failure to timely deliver certificates representing
shares of Common Stock upon exercise of the Warrant as required pursuant to the
terms hereof.

        (b)   If this Warrant shall have been exercised in part, the Company
shall, at the time of delivery of the certificate or certificates representing
Warrant Shares, deliver to the Holder a new Warrant evidencing the rights of the
Holder to purchase the unpurchased Warrant Shares called for by this Warrant,
which new Warrant shall in all other respects be identical with this Warrant.

        (c)   Notwithstanding anything herein to the contrary, in no event shall
the Holder be permitted to exercise this Warrant for Warrant Shares to the
extent that (i) the number of shares of Common Stock beneficially owned by such
Holder, together with any affiliate thereof (other than Warrant Shares issuable
upon exercise of this Warrant) plus (ii) the number of Warrant Shares issuable
upon exercise of this Warrant, would be equal to or exceed 9.9999% of the number
of shares of Common Stock then issued and outstanding, including shares issuable
upon exercise of

2

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

this Warrant held by such Holder after application of this Section 3(c). As used
herein, beneficial ownership shall be determined in accordance with
Section 13(d) of the Exchange Act and the rules promulgated thereunder. To the
extent that the limitation contained in this Section 3(c) applies, the
determination of whether this Warrant is exercisable (in relation to other
securities owned by the Holder) and of which portion of this Warrant is
exercisable shall be in the sole discretion of such Holder, and the submission
of a Notice of Exercise shall be deemed to be such Holder's determination of
whether this Warrant is exercisable (in relation to other securities owned by
such Holder) and of which portion of this Warrant is exercisable, in each case
subject to such aggregate percentage limitation, the Company shall be entitled
to rely on, and shall have no obligation to verify or confirm the accuracy of
such determination. Nothing contained herein shall be deemed to restrict the
right of a Holder to exercise this Warrant into Warrant Shares at such time as
such exercise will not violate the provisions of this Section 3(c). The
provisions of this Section 3(c) may be waived by the Holder upon, at the
election of the Holder, not less than 61 days' prior notice to the Company, and
the provisions of this Section 3(c) shall continue to apply until such 61st day
(or such later date, as determined by the Holder, as may be specified in such
notice of waiver). No exercise of this Warrant in violation of this Section 3(c)
but otherwise in accordance with this Warrant shall affect the status of the
Warrant Shares as validly issued, fully-paid and nonassessable.

        (d)   If, but only if, at any time after one year from the date of
issuance of this Warrant there is no effective Registration Statement
registering the resale of the Warrant Shares by the Holder, this Warrant may
also be exercised at such time by means of a "cashless exercise" in which the
Holder shall be entitled to receive a certificate for the number of Warrant
Shares equal to the quotient obtained by dividing [(A-B) (X)] by (A), where:

        (A) = the Closing Price on the Trading Day preceding the date of such
election;

        (B) = the Exercise Price of the Warrants, as adjusted; and

        (X) = the total number of Warrant Shares with respect to which this
Warrant is then being exercised.

        4.    No Fractional Shares or Scrip.    No fractional shares or scrip
representing fractional shares shall be issued upon the exercise of this
Warrant. As to any fraction of a share which Holder would otherwise be entitled
to purchase upon such exercise, the Company shall pay a cash adjustment in
respect of such final fraction in an amount equal to such fraction multiplied by
the Exercise Price.

        5.    Charges, Taxes and Expenses.    Issuance of certificates for
Warrant Shares shall be made without charge to the Holder for any issue or
transfer tax or other incidental expense in respect of the issuance of such
certificate, all of which taxes and expenses shall be paid by the Company, and
such certificates shall be issued in the name of the Holder or in such name or
names as may be directed by the Holder; provided, however, that in the event
certificates for Warrant Shares are to be issued in a name other than the name
of the Holder, this Warrant when surrendered for exercise shall be accompanied
by the Assignment Form attached hereto duly executed by the Holder; and the
Company may require, as a condition thereto, the payment of a sum sufficient to
reimburse it for any transfer tax incidental thereto.

        6.    Closing of Books.    The Company will not close its stockholder
books or records in any manner which prevents the timely exercise of this
Warrant, pursuant to the terms hereof.

        7.    Transfer, Division and Combination.    

        (a)   Subject to compliance with any applicable securities laws and the
conditions set forth in Sections 1 and 7(e) hereof and to the provisions of
Section 4.1 of the Purchase Agreement, this Warrant and all rights hereunder are
transferable, in whole or in part, upon surrender of this Warrant at the Trading
office of the Company, together with a written assignment of this Warrant
substantially in the form attached hereto duly executed by the Holder or its
agent or attorney and

3

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

funds sufficient to pay any transfer taxes payable upon the making of such
transfer. Upon such surrender and, if required, such payment, the Company shall
execute and deliver a new Warrant or Warrants in the name of the assignee or
assignees and in the denomination or denominations specified in such instrument
of assignment, and shall issue to the assignor a new Warrant evidencing the
portion of this Warrant not so assigned, and this Warrant shall promptly be
cancelled. A Warrant, if properly assigned, may be exercised by a new holder for
the purchase of Warrant Shares without having a new Warrant issued.

        (b)   This Warrant may be divided or combined with other Warrants upon
presentation hereof at the aforesaid office of the Company, together with a
written notice specifying the names and denominations in which new Warrants are
to be issued, signed by the Holder or its agent or attorney. Subject to
compliance with Section 7(a), as to any transfer which may be involved in such
division or combination, the Company shall execute and deliver a new Warrant or
Warrants in exchange for the Warrant or Warrants to be divided or combined in
accordance with such notice.

        (c)   The Company shall prepare, issue and deliver at its own expense
(other than transfer taxes) the new Warrant or Warrants under this Section 7.

        (d)   The Company agrees to maintain, at its aforesaid office, books for
the registration and the registration of transfer of the Warrants.

        (e)   If, at the time of the surrender of this Warrant in connection
with any transfer of this Warrant, the transfer of this Warrant shall not be
registered pursuant to an effective registration statement under the Securities
Act and under applicable state securities or blue sky laws, the Company may
require, as a condition of allowing such transfer (i) that the Holder or
transferee of this Warrant, as the case may be, furnish to the Company a written
opinion of counsel (which opinion shall be in form, substance and scope
customary for opinions of counsel in comparable transactions) to the effect that
such transfer may be made without registration under the Securities Act and
under applicable state securities or blue sky laws, (ii) that the holder or
transferee execute and deliver to the Company an investment letter in form and
substance acceptable to the Company and (iii) that the transferee be an
"accredited investor" as defined in Rule 501(a) promulgated under the Securities
Act.

        8.    No Rights as Shareholder until Exercise.    This Warrant does not
entitle the Holder to any voting rights or other rights as a shareholder of the
Company prior to the exercise hereof. Upon the surrender of this Warrant and the
payment of the aggregate Exercise Price (or by means of a cashless exercise),
the Warrant Shares so purchased shall be and be deemed to be issued to such
Holder as the record owner of such shares as of the close of business on the
later of the date of such surrender or payment.

        9.    Loss, Theft, Destruction or Mutilation of Warrant.    The Company
covenants that upon receipt by the Company of evidence reasonably satisfactory
to it of the loss, theft, destruction or mutilation of this Warrant or any stock
certificate relating to the Warrant Shares, and in case of loss, theft or
destruction, of indemnity or security reasonably satisfactory to it (which, in
the case of the Warrant, shall not include the posting of any bond), and upon
surrender and cancellation of such Warrant or stock certificate, if mutilated,
the Company will make and deliver a new Warrant or stock certificate of like
tenor and dated as of such cancellation, in lieu of such Warrant or stock
certificate.

        10.    Saturdays, Sundays, Holidays, etc.    If the last or appointed
day for the taking of any action or the expiration of any right required or
granted herein shall be a Saturday, Sunday or a legal holiday, then such action
may be taken or such right may be exercised on the next succeeding day not a
Saturday, Sunday or legal holiday.

        11.    Adjustments of Exercise Price and Number of Warrant
Shares.    The number and kind of securities purchasable upon the exercise of
this Warrant and the Exercise Price shall be subject to adjustment from time to
time upon the happening of any of the following. In case the Company shall

4

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

(i) pay a dividend in shares of Common Stock or make a distribution in shares of
Common Stock to holders of its outstanding Common Stock, (ii) subdivide its
outstanding shares of Common Stock into a greater number of shares,
(iii) combine its outstanding shares of Common Stock into a smaller number of
shares of Common Stock, or (iv) issue any shares of its capital stock in a
reclassification of the Common Stock, then the number of Warrant Shares
purchasable upon exercise of this Warrant immediately prior thereto shall be
adjusted so that the Holder shall be entitled to receive the kind and number of
Warrant Shares or other securities of the Company which it would have owned or
have been entitled to receive had such Warrant been exercised in advance
thereof. Upon each such adjustment of the kind and number of Warrant Shares or
other securities of the Company which are purchasable hereunder, the Holder
shall thereafter be entitled to purchase the number of Warrant Shares or other
securities resulting from such adjustment at an Exercise Price per Warrant Share
or other security obtained by multiplying the Exercise Price in effect
immediately prior to such adjustment by the number of Warrant Shares purchasable
pursuant hereto immediately prior to such adjustment and dividing by the number
of Warrant Shares or other securities of the Company resulting from such
adjustment. An adjustment made pursuant to this paragraph shall become effective
immediately after the effective date of such event retroactive to the record
date, if any, for such event.

        12.    Reclassification, Merger, Consolidation or Disposition of
Assets.    In case the Company shall reclassify its Common Stock, consolidate or
merge with or into another corporation (where the Company is not the surviving
corporation), or sell, transfer or otherwise dispose of all or substantially all
its property, assets or business to another corporation and, pursuant to the
terms of such reclassification, merger, consolidation or disposition of assets,
shares of common stock of the successor or acquiring corporation, or any cash,
shares of stock or other securities or property of any nature whatsoever
(including warrants or other subscription or purchase rights) in addition to or
in lieu of common stock of the successor or acquiring corporation ("Other
Property"), are to be received by or distributed to the holders of Common Stock
of the Company, then the Holder shall have the right thereafter to receive, upon
exercise of this Warrant, the number of shares of common stock of the successor
or acquiring corporation and Other Property receivable upon or as a result of
such reclassification, merger, consolidation or disposition of assets by a
holder of the total number of shares of Common Stock for which this Warrant is
exercisable immediately prior to such event. In case of any such
reclassification, merger, consolidation or disposition of assets, the successor
or acquiring corporation (if other than the Company) shall expressly assume the
due and punctual observance and performance of each and every covenant and
condition of this Warrant to be performed and observed by the Company and all
the obligations and liabilities hereunder, subject to such modifications as may
be deemed appropriate (as determined in good faith by resolution of the Board of
Directors of the Company) in order to provide for adjustments of Warrant Shares
for which this Warrant is exercisable which shall be as nearly equivalent as
practicable to the adjustments provided for in this Section 12. For purposes of
this Section 12, "common stock of the successor or acquiring corporation" shall
include stock of such corporation of any class which is not preferred as to
dividends or assets over any other class of stock of such corporation and which
is not subject to redemption and shall also include any evidences of
indebtedness, shares of stock or other securities which are convertible into or
exchangeable for any such stock, either immediately or upon the arrival of a
specified date or the happening of a specified event and any warrants or other
rights to subscribe for or purchase any such stock. The foregoing provisions of
this Section 12 shall similarly apply to successive reorganizations,
reclassifications, mergers, consolidations or disposition of assets.

5

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

        13.    Voluntary Adjustment by the Company.    The Company may at any
time during the term of this Warrant reduce the then current Exercise Price to
any amount and for any period of time deemed appropriate by the Board of
Directors of the Company.

        14.    Notice of Adjustment.    Whenever the number of Warrant Shares or
number or kind of securities or other property purchasable upon the exercise of
this Warrant or the Exercise Price is adjusted, as herein provided, the Company
shall give notice thereof to the Holder, which notice shall state the number of
Warrant Shares (and other securities or property) purchasable upon the exercise
of this Warrant and the Exercise Price of such Warrant Shares (and other
securities or property) after such adjustment, setting forth a brief statement
of the facts requiring such adjustment and setting forth the computation by
which such adjustment was made.

        15.    Notice of Corporate Action.    If at any time:

        (a)   the Company shall take a record of the holders of its Common Stock
for the purpose of entitling them to receive (i) a dividend or other
distribution, or (ii) any right to subscribe for or purchase any evidences of
its indebtedness, any shares of stock of any class or any other securities or
property, or

        (b)   there shall be any reclassification of the Common Stock of the
Company or any consolidation or merger of the Company with, or any sale,
transfer or other disposition of all or substantially all the property, assets
or business of the Company to, another corporation or,

        (c)   there shall be a voluntary or involuntary dissolution, liquidation
or winding up of the Company;

then, in any one or more of such cases, the Company shall give to Holder (i) at
least 20 days' prior written notice of the date on which a record date shall be
selected for such dividend, distribution or right or for determining rights to
vote in respect of any such reclassification, merger, consolidation, sale,
transfer, disposition, liquidation or winding up, and (ii) in the case of any
such reclassification, merger, consolidation, sale, transfer, disposition,
dissolution, liquidation or winding up, at least 20 days' prior written notice
of the date when the same is anticipated to take place (which date can be the
earliest date on which all conditions precedent to the consummation of such
transaction can reasonably be anticipated to be satisfied). Such notice in
accordance with the foregoing clause also shall specify the date on which the
holders of Common Stock are expected to receive such dividend, distribution or
right, and the amount and character thereof. Each such written notice shall be
sufficiently given if addressed to the Holder at the last address of the Holder
appearing on the books of the Company and delivered in accordance with
Section 17(d).

        16.    Authorized Shares.    The Company covenants that during the
period the Warrant is outstanding, it will reserve from its authorized and
unissued Common Stock a sufficient number of shares to provide for the issuance
of the Warrant Shares upon the exercise of any purchase rights under this
Warrant. The Company further covenants that its issuance of this Warrant shall
constitute full authority to its officers who are charged with the duty of
executing stock certificates to execute and issue the necessary certificates for
the Warrant Shares upon the exercise of the purchase rights under this Warrant.
The Company will take all such reasonable action as may be necessary to assure
that such Warrant Shares may be issued as provided herein without violation of
any applicable law or regulation, or of any requirements of the Trading Market
upon which the Common Stock may be listed.

        Except and to the extent as waived or consented to by the Holder, the
Company shall not by any action, including, without limitation, amending its
certificate of incorporation or through any reorganization, transfer of assets,
consolidation, merger, dissolution, issue or sale of securities or any other
voluntary action, intentionally avoid or seek to avoid the observance or
performance of any of the express terms of this Warrant, but will at all times
in good faith assist in the carrying out of all such terms. Without limiting the
generality of the foregoing, the Company will (a) not increase the par value

6

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

of any Warrant Shares above the amount payable therefor upon such exercise
immediately prior to such increase in par value, (b) take all such action as may
be necessary or appropriate in order that the Company may validly and legally
issue fully paid and nonassessable Warrant Shares upon the exercise of this
Warrant, and (c) use commercially reasonable efforts to obtain all such
authorizations, exemptions or consents from any public regulatory body having
jurisdiction thereof as may be necessary to enable the Company to perform its
obligations under this Warrant.

        Before taking any action which would result in an adjustment in the
number of Warrant Shares for which this Warrant is exercisable or in the
Exercise Price, the Company shall obtain all such authorizations or exemptions
thereof, or consents thereto, as may be necessary from any public regulatory
body or bodies having jurisdiction thereof.

        17.    Miscellaneous.    

        (a)   Jurisdiction.    This Warrant shall constitute a contract under
the laws of New York, without regard to its conflict of law, principles or
rules.

        (b)   Restrictions.    The Holder acknowledges that the Warrant Shares
acquired upon the exercise of this Warrant, if not registered, will have
restrictions upon resale imposed by state and federal securities laws.

        (c)   Nonwaiver and Expenses.    No course of dealing or any delay or
failure to exercise any right hereunder on the part of the Holder shall operate
as a waiver of such right or otherwise prejudice the Holder's rights, powers or
remedies, notwithstanding all rights hereunder terminate on the Termination
Date. If the Company willfully and knowingly fails to comply with any provision
of this Warrant, which results in any material damages to the Holder, the
Company shall pay to the Holder such amounts as shall be sufficient to cover any
costs and expenses including, but not limited to, reasonable attorneys' fees,
including those of appellate proceedings, incurred by the Holder in collecting
any amounts due pursuant hereto or in otherwise enforcing any of its rights,
powers or remedies hereunder.

        (d)   Notices.    Any notice, request or other document required or
permitted to be given or delivered to the Holder by the Company shall be
delivered in accordance with the notice provisions of the Purchase Agreement;
provided that upon any permitted assignment of this Warrant, the assignee shall
promptly provide the Company with its contact information.

        (e)   Limitation of Liability.    No provision hereof, in the absence of
any affirmative action by the Holder to exercise this Warrant or purchase
Warrant Shares, and no enumeration herein of the rights or privileges of Holder,
shall give rise to any liability of Holder for the purchase price of any Common
Stock or as a stockholder of the Company, whether such liability is asserted by
the Company or by creditors of the Company.

        (f)    Remedies.    The Holder, in addition to being entitled to
exercise all rights granted by law, including recovery of damages, will be
entitled to specific performance of its rights under this Warrant. The Company
agrees that monetary damages would not be adequate compensation for any loss
incurred by reason of a breach by it of the provisions of this Warrant and
hereby agrees to waive the defense in any action for specific performance that a
remedy at law would be adequate.

        (g)   Successors and Assigns.    Subject to applicable securities laws,
this Warrant and the rights and obligations evidenced hereby shall inure to the
benefit of and be binding upon the successors of the Company and the successors
and permitted assigns of the Holder. The provisions of this Warrant are intended
to be for the benefit of all Holders from time to time of this Warrant and shall
be enforceable by any such Holder or holder of Warrant Shares.

        (h)   Amendment.    This Warrant may be modified or amended or the
provisions hereof waived with the written consent of the Company and the Holder.

7

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

        (i)    Severability.    Wherever possible, each provision of this
Warrant shall be interpreted in such manner as to be effective and valid under
applicable law, but if any provision of this Warrant shall be prohibited by or
invalid under applicable law, such provision shall be ineffective to the extent
of such prohibition or invalidity, without invalidating the remainder of such
provisions or the remaining provisions of this Warrant.

        (j)    Headings.    The headings used in this Warrant are for the
convenience of reference only and shall not, for any purpose, be deemed a part
of this Warrant.

********************

8

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

        IN WITNESS WHEREOF, the Company has caused this Warrant to be executed
by its officer thereunto duly authorized.

Dated: July 1, 2003           Avant Immunotherapeutics, Inc.
 
 
By:
/s/  UNA S. RYAN      

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

Name: Una S. Ryan, Ph.D.
Title: President and Chief Executive Officer

9

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

NOTICE OF EXERCISE

To: AVANT Immunotherapeutics, Inc.

        (1)   The undersigned hereby elects to purchase            Warrant
Shares of AVANT Immunotherapeutics, Inc. pursuant to the terms of the attached
Warrant (only if exercised in full), and tenders herewith payment of the
exercise price in full, together with all applicable transfer taxes, if any.

        (2)   Payment shall take the form of (check applicable box):

[    ] in lawful money of the United States; or

[    ] the cancellation of such number of Warrant Shares as is necessary, in
accordance with the formula set forth in subsection 3(d), to exercise this
Warrant with respect to the maximum number of Warrant Shares purchasable
pursuant to the cashless exercise procedure set forth in subsection 3(d).

        (3)   Please issue a certificate or certificates representing said
Warrant Shares in the name of the undersigned or in such other name as is
specified below:

_________________________

The Warrant Shares shall be delivered to the following:

_________________________

_________________________

_________________________

        (4)    Accredited Investor.    The undersigned is an "accredited
investor" as defined in Regulation D promulgated under the Securities Act of
1933, as amended.

    [PURCHASER]
 
 
By:
       

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

Name:
Title:
 
 
Dated:
       

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

10

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

ASSIGNMENT FORM

(To assign the foregoing warrant, execute
this form and supply required information.
Do not use this form to exercise the warrant.)

        FOR VALUE RECEIVED, the foregoing Warrant and all rights evidenced
thereby are hereby assigned to

___________________________________ whose address is

______________________________________________________.

_____________________________________________________

                                      Dated: _______________, _____

Holder's Signature: _______________________________

Holder's Address: ________________________________

________________________________

Signature Guaranteed: ____________________________________

NOTE: The signature to this Assignment Form must correspond with the name as it
appears on the face of the Warrant, without alteration or enlargement or any
change whatsoever, and must be guaranteed by a bank or trust company. Officers
of corporations and those acting in a fiduciary or other representative capacity
should file proper evidence of authority to assign the foregoing Warrant.

11

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

QuickLinks

COMMON STOCK PURCHASE WARRANT To Purchase 444,444 Shares of Common Stock of
AVANT Immunotherapeutics, Inc.
NOTICE OF EXERCISE
ASSIGNMENT FORM