<PAGE>   1

                                                                     EXHIBIT 4.3


                                ANTIGENICS L.L.C.

                             SUBSCRIPTION AGREEMENT



Antigenics L.L.C.
630 Fifth Avenue
Suite 2170
New York, New York  10111

Ladies and Gentlemen:

     1.   SUBSCRIPTION FOR UNITS. The undersigned, intending to be legally
bound, hereby irrevocably applies to purchase from Antigenics L.L.C., a limited
liability company organized under the Delaware Limited Liability Company Act
(the "LLC"), the number of Units (as defined below) indicated on the signature
page hereof and to be admitted into the LLC as a member as and to the extent
provided herein and in the Private Placement Memorandum dated August __, 1999
(which Private Placement Memorandum, including all amendments thereof and
supplements and exhibits thereto, is herein referred to as the "Memorandum").
Each Unit shall represent a percentage equity interest in the LLC determined
according to the following formula:

                                        $2,401.86
                   Percentage  =   ---------------------
                                    $250,000,000 + A

where "A" equals the aggregate gross proceeds to the LLC in the offering
contemplated by the Memorandum assuming no exercise of any of the Warrants (as
defined below).

This subscription is submitted to the LLC in accordance with and subject to the
terms and conditions described in this Subscription Agreement and the form of
Amended and Restated Limited Liability Company Agreement, dated as of September
10, 1998, as amended by the Amendment Agreement, dated as of December 22, 1998
(the "LLC Agreement"), attached as Exhibits A-1 and A-2 hereto. Notwithstanding
any other provision of the LLC Agreement, if all or substantially all the assets
of the LLC or the outstanding equity interests in the LLC shall be transferred
(by sale, merger or otherwise) to a successor corporation (the "Successor
Corporation") in exchange for securities of such Successor Corporation and such
exchange is made to facilitate a Proposed Registration (as described in Section
7) of the same or any other class of securities of the Successor Corporation,
the undersigned agrees that it will be entitled to receive in such exchange, as
full payment for its Units, a percentage of each class of securities issued by
the Successor Corporation in such exchange equal to the percentage derived by
dividing the total number of Units owned by the undersigned at the time of such
exchange by the total outstanding Units of the LLC at such time. THE SIGNATURE
OF THE UNDERSIGNED HERETO CONSTITUTES AUTHORIZATION OF THE EXECUTION ON BEHALF
OF THE UNDERSIGNED OF THE LLC AGREEMENT OR AN AMENDMENT THERETO FOR THE PURPOSE
OF ADMITTING THE UNDERSIGNED AS A MEMBER OF THE LLC. See Section 4 below.

<PAGE>   2


     2.   WARRANTS. In connection with the purchase of any Units subscribed for
in Section 1 (the "Initial Units"), the LLC will issue to the undersigned a
warrant (a "Warrant") exercisable for a number of additional Units equal to 5%
of the number of Initial Units, rounded down to the nearest whole number. The
Warrant shall entitle the undersigned to purchase Units at any time until
September 30, 2002, shall have a per Unit exercise price of $2,401.86, and shall
be in substantially the form of Exhibit B hereto.

     3.   AMOUNT AND METHOD OF PAYMENT. The undersigned encloses herewith a
check made payable to "Antigenics L.L.C.", or contemporaneously with the
undersigned's delivery of this Subscription Agreement, is paying by wire
transfer to the account of Antigenics L.L.C., the consideration (the "Purchase
Price") required to purchase the Units subscribed for hereunder, in the amount
of $2,401.86 for each Unit subscribed for, which represents payment in full for
the subscription. The minimum purchase is 416 Units; provided, however, the LLC
may decide, in its discretion, to accept subscriptions for less than 416 Units
or to limit any subscription to 4,163 Units.

     4.   ADMISSION TO LLC; POWER OF ATTORNEY. An investor, whose subscription
agreement is accepted by the LLC and whose payment of the purchase price for the
Units to be purchased by the undersigned is received by the LLC, will become a
party to the LLC Agreement at such time as the investor's admission to the LLC
is reflected in the books and records of the LLC (the "LLC Closing"). The
undersigned has received and read a copy of the form of Amended and Restated
Limited Liability Company Agreement and the Amendment Agreement thereto attached
as Exhibits A-1 and A-2 hereto and accepts and agrees to be bound by all of the
terms thereof and to perform all obligations therein imposed upon an investor
with respect to the equity interest included in each Unit purchased, and any
equity interest acquired upon exercise of a Warrant.

          If the subscription is rejected as provided in Section 5 hereof, the
undersigned's subscription shall be void and all funds received from the
undersigned, together with any interest earned thereon, shall be promptly
returned to the undersigned.

          The undersigned, by the execution and delivery of this Subscription
Agreement, hereby irrevocably constitutes and appoints each of the Chairman of
the Board of Managers and Chief Executive Officer and the Vice Chairman of the
Board of Managers of the LLC with full power of substitution, as the true and
lawful agent and attorney-in-fact for the undersigned and authorizes and
empowers such attorney, in the name, place, and stead of the undersigned, to
make, execute, deliver, acknowledge, swear to, file and record in all necessary
or appropriate places the LLC Agreement and such other documents and instruments
(including, without limitation, the LLC Agreement and amendments and
restatements of the LLC Agreement in accordance with the terms of the LLC
Agreement), and to take such other actions as may be necessary or appropriate to
carry out the LLC Agreement. The Power of Attorney granted hereby shall be
deemed to be coupled with an interest, shall be irrevocable and shall survive
and shall not be affected by the subsequent death, disability, incapacity,
insolvency or bankruptcy of the undersigned or the transfer of the equity
interest of the undersigned until such time as the transferee shall have been
admitted to the Limited Liability Company as a member or the transfer of any
portion of the equity interest.

<PAGE>   3

     5.   ACCEPTANCE OF SUBSCRIPTION.

          (a)  The undersigned understands and agrees that the LLC, in its sole
discretion, reserves the right to accept or reject this and any other
subscription for Units in whole or in part at any time prior to the sale of such
Units, notwithstanding prior receipt by the undersigned of notice of acceptance.

          (b)  In the event that this subscription is rejected in whole or in
part, or if the sale of the Units is not consummated for any reason (in which
event this subscription shall be deemed to be rejected), the LLC shall promptly
cause the return of the Purchase Price to the undersigned, and this Subscription
Agreement shall thereafter have no force or effect to that extent.

     6.   REPRESENTATIONS AND WARRANTIES. The undersigned hereby acknowledges,
represents, warrants to, and agrees with the LLC as follows:

          (a)  The undersigned understands that the offering and sale of the
Units and Warrants is intended to be exempt from registration under the
Securities Act of 1933, as amended (the "Securities Act"), by virtue of Section
4(2) of the Securities Act and the provisions of Rule 506 of Regulation D
promulgated thereunder and, in accordance therewith and in furtherance thereof,
the undersigned represents and warrants to and agrees with the LLC as follows:

               (i)  The undersigned has received the Memorandum and the LLC
Agreement, has carefully reviewed each and understands the information contained
therein and information otherwise provided to the undersigned in writing by the
LLC relating to this investment and has had the opportunity to show to, and
discuss with, the undersigned's attorney, accountant and financial advisor, all
such information;

               (ii) The undersigned understands that all other documents,
records, and books pertaining to this investment have been made available for
inspection by the undersigned, the undersigned's attorney, the undersigned's
accountant and the undersigned's financial advisor;

               (iii) The undersigned and/or the undersigned's advisor(s) have
had a reasonable opportunity to ask questions of and receive information and
answers from a person or persons acting on behalf of the LLC concerning the
offering of the Units and, as the undersigned has deemed necessary, to verify
the information contained in the Memorandum and all such questions have been
answered and all such information has been provided to the full satisfaction of
the undersigned;

               (iv) No oral or written representations have been made or oral or
written information furnished to the undersigned or the undersigned's advisor(s)
in connection with the offering of the Units which were in any way inconsistent
with or in addition to the information stated in the Memorandum;

               (v)  The undersigned is not subscribing for a Unit as a result of
or subsequent to any advertisement, article, notice, or other communication
published in any


                                       3
<PAGE>   4

newspaper, magazine, or similar media or broadcast over television or radio, or
presented at any seminar or meeting, or any solicitation of a subscription by a
person not previously known to the undersigned in connection with investments in
securities generally;

               (vi) If the undersigned is a natural person, the undersigned has
reached the age of majority in the state in which the undersigned resides, has
adequate means of providing for the undersigned's current needs and personal
contingencies, is able to bear the substantial economic risks of an investment
in the LLC for an indefinite period of time, has no need for liquidity in such
investment, and could afford a complete loss of such investment;

               (vii) The undersigned has such knowledge and experience in
financial, tax and business matters so as to enable the undersigned to utilize
the information made available to the undersigned in connection with the
offering of the Units in order to evaluate the merits and risks of an investment
in the LLC and to make an informed investment decision with respect thereto and,
therefore, he is not relying upon the advice of a purchaser representative in
making a final investment decision to purchase Units;

               (viii) The undersigned is not relying on the LLC with respect to
the tax and other economic considerations of the undersigned relating to this
investment. In regard to such considerations, the investor has relied on the
advice of, or has consulted with, only the undersigned's own professional
advisors who are unaffiliated with and who are not directly or indirectly
compensated by the LLC or any affiliate;

               (ix) The undersigned understands that the undersigned may be
subject to taxes with respect to allocations of income and distributions with
respect to the undersigned's equity interest in the LLC and that pursuant to the
provisions of the LLC Agreement, among other things, the LLC shall be entitled
to deduct, to withhold, and/or to pay any and all taxes and withholdings, and
all interest, penalties, additions to tax, and similar liabilities in connection
therewith or attributable thereto. (See Section 8.6 of the LLC Agreement). It
shall be the policy of the LLC to consider the tax liabilities of equity
interest holders with respect to allocations of income to their equity interests
in connection with the LLC's determinations from time to time regarding whether
to make any distributions to equity interest holders and the extent thereof. The
undersigned further understands that no assurance can be made that any such
distributions will be made, or that, if made, will be in amounts sufficient to
enable equity interest holders to pay all such taxes;

               (x)  The undersigned is acquiring the Units and Warrants solely
for the undersigned's own account as principal, for investment purposes only and
not with a view to the resale or distribution thereof, in whole or in part, and
no other person has a direct or indirect beneficial interest in such equity
interest;

               (xi) The undersigned will not sell or otherwise transfer the
Units or Warrants without registration under the Securities Act or an exemption
therefrom or if such sale or transfer would violate any provision of the LLC
Agreement, and fully understands and agrees that the undersigned must bear the
economic risk of the undersigned's purchase for an indefinite period of time
because, among other reasons, neither the Units nor the Warrants have been
registered under the Securities Act or under the securities laws of certain
states and, therefore,


                                       4

<PAGE>   5


cannot be resold, pledged, assigned, or otherwise disposed of unless they are
subsequently registered under the Securities Act and under the applicable
securities laws of such states or unless an exemption from such registration is
available. Except as otherwise set forth herein, the undersigned further
understands that the LLC is not under any obligation to register the Units or
Warrants on the undersigned's behalf or to assist the undersigned in complying
with any exemption from registration; and

               (xii) The undersigned understands that sales or transfers of the
Units are further restricted by the provisions of the LLC Agreement and that
sales or transfers of securities are restricted under the Securities Act and
under certain state securities laws.

          (b)  The undersigned recognizes that an investment in the LLC involves
a high degree of risk. Among other considerations in this regard,

               (i)  no Federal or state agency has passed upon the Units or the
Warrants or made any finding or determination as to the fairness of this
investment;

               (ii) there is no established market for the Units or the
Warrants; and it is unlikely that a public market for such securities will
develop.

          (c)  If the undersigned is a corporation, partnership, trust, or other
entity, it is authorized and qualified to become a member in, and make its
capital contributions to, the LLC, and the person signing this Subscription
Agreement on behalf of such entity has been duly authorized by such entity to do
so.

          (d)  If the undersigned is purchasing the Units subscribed for hereby
in a representative or fiduciary capacity, the representations and warranties
contained herein (and in any other written statement or document delivered in
connection herewith) shall be deemed to have been made on behalf of the person
or persons for whom such equity interest is being purchased.

          (e)  If the undersigned is a natural person, the undersigned is an
"accredited investor" within the meaning of Rule 501 under the Securities Act
because the undersigned (i) had individual income in excess of $200,000 in each
of the last two calendar years and the undersigned reasonably expects to have
individual income in excess of $200,000 in the current calendar year; and/or
(ii) the undersigned had joint income with the spouse of the undersigned in
excess of $300,000 in each of the last two calendar years and the undersigned
reasonably expects to have joint income in excess of $300,000 in the current
calendar year; and/or (iii) the undersigned has an individual net worth, or the
spouse of the undersigned and the undersigned have a joint net worth, in excess
of $1,000,000. If the undersigned is an entity, the undersigned is an
"accredited investor" within the meaning of Rule 501 under the Securities Act
because (i) the undersigned was not formed for the specific purpose of acquiring
the securities offered and has total assets in excess of $5,000,000, or (ii) all
of the equity owners of the undersigned are "accredited investors" pursuant to
the preceding clause (i) and/or the preceding sentence.

          (f)  All information which the undersigned has heretofore furnished
and furnishes herewith to the LLC, including, without limitation, the
certification as to the undersigned's status as an "accredited investor" within
the meaning of Rule 501 under the


                                       5
<PAGE>   6


Securities Act and applicable state securities laws, and any other information
with respect to the undersigned's financial position and business experience set
forth herein, and any representations contained herein, is correct and complete
as of the date of this Subscription Agreement, and if there should be any
material change in such information prior to the undersigned's admission to the
LLC as a member, the undersigned will immediately furnish such revised or
corrected information to the LLC.

          (g)  Within five days after receipt of a request from the LLC, the
undersigned hereby agrees to provide such information and to execute and deliver
such documents as may reasonably be necessary to comply with any and all laws
and ordinances to which the LLC is subject.

          (h)  The foregoing representations, warranties, and agreements,
together with all other representations and warranties made or given by the
undersigned to the LLC in any other written statement or document delivered in
connection with the transactions contemplated hereby, shall be true and correct
in all respects on and as of the date of the undersigned's admission to the LLC
as a member as if made on and as of such date and shall survive such date.

     7.   PIGGYBACK REGISTRATION RIGHTS.

          (a)  If prior to the second anniversary of the LLC Closing, the LLC or
its successor (the "Company") proposes to register under the Securities Act (a
"Proposed Registration") the LLC equity interests or shares of common stock or
other securities into which such equity interests have been exchanged in a
reorganization (the "Securities") in connection with the initial public offering
of such Securities (other than a registration on Form S-4 or any successor
form), the Company shall, at such time, promptly give the undersigned written
notice of such Proposed Registration. The undersigned shall have ten (10) days
from its receipt of such notice to deliver to the Company a written request
specifying the amount of Securities purchased pursuant to this Subscription
Agreement or acquired upon exercise of Warrants purchased pursuant to this
Subscription Agreement (including any Securities received in a reorganization)
that the undersigned intends to sell (the "Registrable Securities") and the
undersigned's intended method of distribution. Upon receipt of such request, the
Company shall use reasonable commercial efforts to cause all Registrable
Securities which the Company has been requested to register to be registered
under the Securities Act to the extent necessary to permit their sale or other
disposition in accordance with the intended method of distribution specified in
the request of the undersigned; provided, however, that the Company shall have
the right, prior to the date the applicable registration statement becomes
effective, to postpone or withdraw any Proposed Registration without obligation
to the undersigned. If the Proposed Registration involves an underwriting, the
Company shall not be required to include any Registrable Securities in the
underwritten portion of the offering.

          (b)  In connection with the registration of Registrable Securities
pursuant to this Agreement, the Company shall:

               (i)  subject to subsection (iv), keep a registration statement
covering Registrable Securities effective until the earliest of (A) one year
after the effective date thereof, (B) the sale of all Registrable Securities
covered by the registration statement, or (C) the date on


<PAGE>   7


which all the remaining Registrable Securities can be immediately sold to the
public without registration;

               (ii) prepare and file with the Commission such amendments and
supplements to a registration statement covering Registrable Securities and the
prospectus used in connection with such registration statement as may be
necessary to comply with the provisions of the Securities Act and the rules and
regulations thereunder to maintain the effectiveness of the registration
statement;

               (iii) furnish to the undersigned such number of copies of the
prospectus included in a registration statement covering Registrable Securities
as the undersigned may reasonably request in order to facilitate the disposition
of the undersigned's Registrable Securities;

               (iv) notify the undersigned promptly upon the occurrence of any
event or circumstance that, in the reasonable judgment of the Company, makes it
necessary or appropriate to amend or supplement the prospectus included in a
registration statement covering Registrable Securities, and promptly prepare,
file and furnish to the undersigned a reasonable number of copies of such a
supplemented or amended prospectus; provided, however, that the Company may
delay preparing, filing and distributing any such supplement or amendment if the
Company determines that such supplement or amendment could (i) interfere with or
adversely affect the negotiation or completion of a transaction that is being
contemplated by the Company or (ii) involve initial or continuing disclosure
obligations that are not in the best interests of the Company's equityholders at
such time; provided, further, that (x) the Company will give notice of any such
delay prior to such delay, (y) such delay shall not extend for a period of more
than sixty (60) days without the written consent of the undersigned and (z) the
Company may utilize such delay no more than twice in any period of 365
consecutive days; and

               (v)  use reasonable commercial efforts to prevent the issuance of
any stop order or other order suspending the effectiveness of a registration
statement covering Registrable Securities and, if such an order is issued, to
obtain the withdrawal thereof at the earliest possible time and to notify the
undersigned of the issuance of such order and the resolution thereof.

          (c)  In connection with any registration of the Registrable
Securities, the undersigned shall:

               (i)  furnish to the Company such information regarding itself and
the intended method of disposition of Registrable Securities as the Company
shall reasonably request in order to effect the registration thereof; and

               (ii) upon receipt of any notice from the Company of the issuance
of a stop order or a notice under Section 6(b)(iv), immediately discontinue
disposition of Registrable Securities pursuant to the applicable registration
statement until withdrawal of the stop order or receipt of the amended or
supplemented prospectus, as the case may be.

          (d)  With a view to making available to the undersigned the benefits
of Rule 144 under the Securities Act ("Rule 144"), the Company agrees that after
it becomes subject to

                                       7

<PAGE>   8


the reporting requirements under the Securities Exchange Act of 1934, as
amended, and until the second anniversary of the purchase of the Units, it shall
make available adequate public information, as that term is defined in Rule 144.

          (e)  Expenses incurred by the Company in connection with the
registration of Registrable Securities, including registration fees, printer
costs, accounting fees and the fees and disbursements of counsel for the
Company, shall be borne by the Company. The Company shall not be responsible for
underwriting discounts and commissions payable with respect to Registrable
Securities or fees and expenses for attorneys or other advisors to the
undersigned.

     8.   INDEMNIFICATION. The undersigned agrees to indemnify and hold harmless
the LLC and its officers, directors, and Affiliates and each other person, if
any, who controls any thereof, within the meaning of Section 15 of the
Securities Act, against any and all losses, liabilities, claims, damages, and
expenses whatsoever (including, but not limited to, any and all expenses
reasonably incurred in investigating, preparing, or defending against any
litigation commenced or threatened or any claim whatsoever) arising out of or
based upon any false representations or breaches of warranty or breach or
failure by the undersigned to comply with any covenant or agreement made by the
undersigned herein or in any other document furnished by the undersigned to any
of the foregoing in connection with this transaction.

     9.   ADDITIONAL INFORMATION. The undersigned hereby acknowledges and agrees
that the LLC may make or cause to be made such further inquiry and obtain such
additional information as they may deem appropriate, with regard to the
suitability of the undersigned.

     10.  IRREVOCABILITY; BINDING EFFECT. The undersigned hereby acknowledges
and agrees that the subscription hereunder is irrevocable, that the undersigned
is not entitled to cancel, terminate, or revoke this Subscription Agreement or
any agreements of the undersigned thereunder, and that this Subscription
Agreement and such other agreements shall survive the death or disability of the
undersigned and shall be binding upon and inure to the benefit of the parties
and their heirs, executors, administrators, successors, legal representatives,
and assigns. If the undersigned is more than one person, the obligations of the
undersigned hereunder shall be joint and several and the agreements,
representations, warranties, and acknowledgements herein contained shall be
deemed to be made by and be binding upon each such person and his heirs,
executors, administrators, successors, legal representatives, and assigns.

     11.  MODIFICATION. Neither this Subscription Agreement nor any provisions
hereof shall be waived, modified, discharged, or terminated except by an
instrument in writing signed by the party against whom any such waiver,
modification, discharge, or termination is sought.

     12.  NOTICES. Any notice, demand, or other communication which any party
hereto may be required, or may elect, to give to any other party hereunder shall
be sufficiently given if (a) deposited, postage prepaid, in a United States mail
box, stamped registered or certified mail, return receipt requested, addressed
to such address as may be listed on the books of the LLC, or (b) delivered
personally at such address.

     13.  COUNTERPARTS. This Subscription Agreement may be executed through the
use of separate signature pages or in any number of counterparts, and each such
counterpart shall, for


<PAGE>   9


all purposes, constitute one agreement binding on all parties, notwithstanding
that all parties are not signatories to the same counterpart.

     14.  ENTIRE AGREEMENT. This Subscription Agreement contains the entire
agreement of the parties with respect to the subject matter hereof and there are
no representations, covenants or other agreements except as stated or referred
to herein.

     15.  SEVERABILITY. Each provision of this Subscription Agreement is
intended to be severable from every other provision, and the invalidity or
illegality of any portion hereof shall not affect the validity or legality of
the remainder hereof.

     16.  ASSIGNABILITY. This Subscription Agreement is not transferable or
assignable by the undersigned.

     17.  APPLICABLE LAW. This Subscription Agreement shall be governed by and
construed in accordance with the laws of the State of New York as applied to
residents of that State executing contracts wholly to be performed in that
State.

     18.  CHOICE OF JURISDICTION. The parties agree that any action or
proceeding arising, directly, indirectly, or otherwise, in connection with, out
of, or from this Subscription Agreement, any breach hereof or any transaction
covered hereby shall be resolved within the County, City, and State of New York.
Accordingly, the parties consent and submit to the jurisdiction of the United
States federal and state courts located within the County, City, and State of
New York.

     19.  TAXPAYER IDENTIFICATION NUMBER. The undersigned verifies under
penalties of perjury that the Taxpayer Identification Number or Social Security
Number shown below is true, correct, and complete and that the undersigned is
not subject to backup withholding either (a) because the undersigned has not
been notified that it is subject to backup withholding as a result of a failure
to report all interest or dividends or (b) because the Internal Revenue Service
has notified the undersigned that the undersigned is no longer subject to backup
withholding.



  ------------------------------------------------------------------------
  Number of Units for                             $___________
  (minimum of _______)
  ------------------------------------------------------------------------
  Aggregate Purchase Price                        $___________
  ------------------------------------------------------------------------


  Names in which the Units are to be registered:

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

  Federal Taxpayer I.D. or Social Security Number (applicable only to U.S.
  subscribers):

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


                                       9
<PAGE>   10


         IN WITNESS WHEREOF, the undersigned has caused this Subscription
Agreement to be executed this ___ day of ____________, 1999.





Individuals:                        ____________________________________________

                                    Print name: ________________________________



                                    Print name of joint owner, if any below:

                                    ____________________________________________

                                    Print address: _____________________________



Corporation or other entity:        Print name of corporation or other entity

                                    By:_________________________________________

Authorized Signatory

                                    Print name: ________________________________

                                    Print address:______________________________



<PAGE>   11


                   THIS PAGE NOT TO BE COMPLETED BY SUBSCRIBER





                             RECEIPT AND ACCEPTANCE





              SUBSCRIPTION ACCEPTED ON _____________________, 1999



                                  ANTIGENICS L.L.C.



                                  By:______________________________________
                                     Garo H. Armen,
                                     Chairman of the Board of Managers and
                                     Chief Executive Officer



                                       11
<PAGE>   12

                       AMENDMENT TO SUBSCRIPTION AGREEMENT


          THIS AMENDMENT (the "Amendment") to the Subscription Agreement related
to the Private Placement Memorandum dated August 31, 1999 (the "Subscription
Agreement") between Antigenics L.L.C., a limited liability company organized
under the Delaware Limited Liability Company Act (the "Company") and the
undersigned, is hereby entered into by the Company and the undersigned.

          In consideration for good and valuable consideration, the receipt and
sufficiency of which are hereby acknowledged, the Company and the undersigned
hereby amend the Subscription Agreement as follows:

1.   All capitalized terms set forth herein shall have the same meaning as set
forth in the Subscription Agreement.

2.   Section 2 of the Subscription Agreement is hereby deleted in its entirety
     and replaced with the following:

               "2. Warrants. In connection with the purchase of any Units
subscribed for in Section 1 (the "Initial Units"), the LLC will issue to the
undersigned a warrant (a "Warrant") exercisable for a number of additional Units
equal to 10% of the number of Initial Units, rounded down to the nearest whole
number. The Warrant shall entitle the undersigned to purchase Units at any time
until September 30, 2002, shall have a per Unit exercise price of $2,401.86, and
shall be in substantially the form of Exhibit B hereto."

3.   Section 7(a) of the Subscription Agreement is hereby deleted in its
     entirety and replaced with the following:

               "7. Piggyback Registration Rights.

               (a) If prior to the second anniversary of the LLC Closing, the
LLC or its successor (the "Company") registers under the Securities Act (a
"Proposed Registration") the LLC equity interests or shares of common stock or
other securities into which such equity interests have been exchanged in a
reorganization (the "Securities") in connection with the initial public offering
of such Securities (other than a registration on Form S-4 or any successor
form), the Company shall, on the 91st day after the effective date of such
registration statement, promptly give the undersigned written notice of such
Proposed Registration. The undersigned shall have ten (10) days from its receipt
of such notice to deliver to the Company a written request specifying the amount
of Securities purchased pursuant to this Subscription Agreement or acquired upon
exercise of Warrants purchased pursuant to this Subscription Agreement
(including any Securities received in a reorganization) that the undersigned
intends to sell (the "Registrable Securities") and the undersigned's intended
method of distribution. Upon receipt of such request, the Company shall use
reasonable commercial efforts to cause all Registrable Securities which the
Company has been requested to register to be registered under the Securities


<PAGE>   13


Act to the extent necessary to permit their sale or other disposition in
accordance with the intended method of distribution specified in the request of
the undersigned; provided, however, that the Company shall have the right, prior
to the date the applicable registration statement becomes effective, to postpone
or withdraw any Proposed Registration without obligation to the undersigned."

4.   Subject to the amendment set forth herein, the remainder of the
     Subscription Agreement shall remain in full force and effect.

5.   This Amendment may be executed in two or more counterparts, each of which
     shall be deemed an original, but all of which together shall constitute one
     and the same instrument.



                                       13

<PAGE>   14


                  IN WITNESS WHEREOF, the undersigned has caused this Amendment
to the Subscription Agreement to be executed this ___ day of ____________, 1999.





                                     ANTIGENICS L.L.C.



                                     By:_____________________________________

                                        Garo H. Armen,

                                        Chairman of the Board of Managers and
                                        Chief Executive Officer








            [THE REMAINDER OF THIS PAGE IS INTENTIONALLY LEFT BLANK.]




                                       14

<PAGE>   15


Individuals:                        ____________________________________________

                                    Print name:_________________________________



                                    Print name of joint owner, if any below:

                                    ____________________________________________

                                    Print address:______________________________



Corporation or other entity:        Print name of corporation or other entity

                                    By:_________________________________________

Authorized Signatory

                                    Print name:_________________________________

                                    Print address:______________________________





                                       15

<PAGE>   16
List of Parties to Subscription Agreement

Blackford Securities
Michael & Pamela Clark
Stephen A. Cohen
Mary & Michael Darling
Delaware Charter Guarantee
Russell S. Reynolds III
Eagle Constellation Fund
Elan Corporation
EM Sterne & DM Moore TTEES
Stephen Feid
Freedale Investments
Frost Nevada
Phillip T. George, M.D.
German American Capital Corp.
H. Leland Getz
Joseph Grano Jr.
Kara Ann Berg Trust
Dr. Ernest Mario
Donald Marron
Mark McInerney
John P. McNiff
Richard Omohundro
James Pinto
Pinto Trust, C.S.
Pinnacle Investments
PNC Bank Delaware
SAC Capital
Stephen Schram
Sigma-Tau Industrie
Farmaceutiche Riunite SpA
Aptafin
Richard Sterne
Carol Swiggett
John Martin Taylor
ThermoElectron
A&A Actien Bank
Dr. Darrick Antell
Michael G. Baldwin
E. Garrett Bewkes Jr.
Blue Star Group
Arthur Castlebaum
Coleman Partnership
Dean C. Colson
Dean C. Colson, IRA
Dominic Corvino
Carmine DeSantis
Ralph A. Daiuto Sr.
Sergio Dompe
Lew Eisenberg
Gibralt Capital Corp
Chris Hart
H.B. Rigs Ltd.
Robert Hoffman
Medison Pharma Ltd.
Medison Tech.
Mark Tzalkovitz
Loren Kramer
Dr. Bernard Lahasky
Andrew J. Lanza
Peter Lawson-Johntson II
Peter Lawson-Johntson
Links LLC
Fredda Levitt
Dr. Ivan Lieberberg
Michael E. Mederrick
William Morrison
John Morse
PaineWebber
Darryl Parmenter
Manny Reiser
SAAD Investment Company Ltd.
John Saraceno
James F. & Virginia Sattler
A. Scurfin-Moratti
N. Lloyd Scurlock
Walter Toombs
Neil & Karen Vaccaro
H. William & Laura Walker
Frank B. White
Joseph E. Wurtz
Strauss Zelnick
Pilar Morales-Arce
E. Gutzwiller & CIE
S. L. Gubel
Interimage
Lloyds
Osa Mayor
Carmen Miranda
Percacer SA
Redwood Investment Ltd.
L. Rispoli
Stanhope S.L.
STH Capital
Olga Subirana
Torreal