<PAGE>

                                  ABGENIX, INC.

                                       and

                           ABGENIX CANADA CORPORATION

                                       and

                         IMMGENICS PHARMACEUTICALS INC.

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                                SUPPORT AGREEMENT

                                November 3, 2000

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                                SUPPORT AGREEMENT

      THIS SUPPORT AGREEMENT (this "Agreement") made as of November 3, 2000
between Abgenix, Inc., a corporation existing under the laws of Delaware
(hereinafter referred to as "Abgenix") and ImmGenics Pharmaceuticals Inc., a
company existing under the laws of British Columbia (hereinafter referred to as
the "Company") and Abgenix Canada Corporation, an unlimited liability company
existing under the laws of Nova Scotia (hereinafter referred to as "Abgenix
Canada").

                                    RECITALS:

      In connection with an acquisition and arrangement agreement (as amended,
supplemented and/or restated, (the "Acquisition Agreement") made as of September
25, 2000 between Abgenix, Abgenix Canada and the Company, the Company is to
issue exchangeable shares (the "Company Special Shares") to all holders of
securities of the Company pursuant to the plan of arrangement contemplated by
the Acquisition Agreement; and

      Pursuant to the Acquisition Agreement, Abgenix, Abgenix Canada and the
Company are required to execute a support agreement substantially in the form of
this Agreement.

      In consideration of the foregoing and the mutual agreements contained
herein (the receipt and adequacy of which are acknowledged), the parties agree
as follows:

                                    ARTICLE 1
                         DEFINITIONS AND INTERPRETATION

1.1 Defined Terms

      Each term denoted herein by initial capital letters and not otherwise
defined herein shall have the meaning ascribed thereto in the rights,
privileges, restrictions and conditions (collectively, the "Share Provisions")
attaching to the Company Special Shares as set forth in the Plan of Arrangement,
unless the context requires otherwise.

1.2 Interpretation Not Affected by Headings

      The division of this Agreement into Articles, sections and other portions
and the insertion of headings are for convenience of reference only and shall
not affect the construction or interpretation of this Agreement. Unless
otherwise indicated, all references to an "Article" or "Section" followed by a
number and/or a letter refer to the specified Article or Section of this
Agreement. The terms "this Agreement", "hereof", "herein" and "hereunder" and
similar expressions refer to this Agreement and not to any particular Article,
section or other portion hereof and include any agreement or instrument
supplementary or ancillary hereto.

1.3 Number, Gender

      Words importing the singular number only shall include the plural and vice
versa. Words importing any gender shall include all genders.
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1.4 Date for any Action

      If any date on which any action is required to be taken under this
Agreement is not a Business Day, such action shall be required to be taken on
the next succeeding Business Day. For the purposes of this Agreement, a
"Business Day" means any day other than a Saturday, a Sunday or any day on which
commercial banks located in the Province of British Columbia, the State of
California or the State of Delaware are authorized or obligated to close.

                                    ARTICLE 2
                      COVENANTS OF Abgenix AND THE COMPANY

2.1 Covenants Regarding Company Special Shares

(1) So long as any Company Special Shares not owned by Abgenix or its Affiliates
are outstanding, Abgenix will:

      (a)   not declare or pay any dividend on the Abgenix Common Shares unless
            (i) it shall cause the Company simultaneously to declare or pay, as
            the case may be, an equivalent dividend (as provided for in the
            Share Provisions) on the Company Special Shares and (ii) the Company
            shall have sufficient money or other assets or authorized but
            unissued securities available to enable the due declaration and the
            due and punctual payment, in accordance with applicable law, of any
            such dividend on the Company Special Shares;

      (b)   advise the Company sufficiently in advance of the declaration by
            Abgenix of any dividend on Abgenix Common Shares and take all such
            other actions as are reasonably necessary, in co-operation with the
            Company to ensure that the respective declaration date, record date
            and payment date for a dividend on the Company Special Shares shall
            be the same as the declaration date, record date and payment date
            for the corresponding dividend on the Abgenix Common Shares;

      (c)   ensure that the record date for any dividend declared on Abgenix
            Common Shares is not less than ten (10) Business Days after the
            declaration date of such dividend;

      (d)   take all such actions and do all such things as are reasonably
            necessary or desirable to enable and permit the Company, in
            accordance with applicable law, to pay and otherwise perform its
            obligations with respect to the satisfaction of the Liquidation
            Amount, the Retraction Price or the Redemption Price in respect of
            each issued and outstanding Company Special Share (other than
            Company Special Shares owned by Abgenix or its Affiliates) upon the
            liquidation, dissolution or winding-up of the Company, the delivery
            of a Retraction Request by a holder of Company Special Shares or a
            redemption of the Company Special Shares by the Company, as the case
            may be, including without limitation all such actions and all such
            things as are necessary or desirable to enable and permit the
            Company to cause to be delivered Abgenix Common Shares to the
            holders of the Company Special Shares in accordance with the
            provisions of Section 5, 6 or 7, as the case may be, of the Share
            Provisions; and


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<PAGE>

      (e)   take all such actions and do all such things as are reasonably
            necessary or desirable to enable and permit Abgenix Canada, in
            accordance with applicable law, to perform its obligations arising
            upon the exercise by it of the Liquidation Call Right, the
            Retraction Call Right or the Redemption Call Right, including
            without limitation all such actions and all such things as are
            necessary or desirable to enable and permit Abgenix Canada to cause
            to be delivered Abgenix Common Shares to the holders of the Company
            Special Shares in accordance with the provisions of the Liquidation
            Call Right, the Retraction Call Right or the Redemption Call Right,
            as the case may be.

(2) Prior to December 31, 2004, Abgenix shall not exercise its vote as a
shareholder to initiate the voluntary liquidation, dissolution or winding up of
the Company nor take any action or omit to take any action that is designed to
result in the liquidation, dissolution or winding up of the Company.

2.2 Segregation of Funds

      Abgenix will cause the Company to deposit a sufficient amount of funds in
a separate account of the Company and segregate a sufficient amount of such
other assets and property as is necessary to enable the Company to pay dividends
when due and to pay or otherwise satisfy its respective obligations under
Section 5, 6 or 7 of the Share Provisions, as applicable.

2.3 Reservation of Abgenix Common Shares

      Abgenix hereby represents, warrants and covenants in favour of the Company
and Abgenix Canada that Abgenix has reserved for issuance and will, at all times
while any Company Special Shares (other than the Company Special Shares held by
Abgenix or its Affiliates) are outstanding, keep available, free from
pre-emptive and other rights, out of its authorized and unissued capital stock
such number of Abgenix Common Shares (or other shares or securities into which
Abgenix Common Shares may be reclassified or changed as contemplated by Section
2.7: (a) as is equal to the sum of: (i) the number of the Company Special Shares
issued and outstanding from time to time and (ii) the number of the Company
Special Shares issuable upon the exercise of all rights to acquire the Company
Special Shares outstanding from time to time, multiplied, in each case, by the
Exchange Ratio; and (b) as are now and may hereafter be required to enable and
permit Abgenix to meet its obligations under the Voting, Exchange and Cash Put
Trust Agreement and under any other security or commitment pursuant to which
Abgenix may now or hereafter be required to issue Abgenix Common Shares, to
enable and permit Abgenix Canada to meet its obligations under each of the
Liquidation Call Right, the Retraction Call Right and the Redemption Call Right
and to enable and permit the Company to meet its obligations hereunder and under
the Share Provisions.

2.4 Notification of Certain Events

      In order to assist Abgenix to comply with its obligations hereunder and to
permit Abgenix Canada to exercise the Liquidation Call Right, the Retraction
Call Right and the Redemption Call Right, the Company will notify Abgenix and
Abgenix Canada of each of the following events at the time set forth below:


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      (a)   in the event of any determination by the Board of Directors of the
            Company to institute voluntary liquidation, dissolution or
            winding-up proceedings with respect to the Company or to effect any
            other distribution of the assets of the Company among its
            shareholders for the purpose of winding up its affairs, at least
            sixty (60) days prior to the proposed effective date of such
            liquidation, dissolution, winding-up or other distribution;

      (b)   immediately, upon the earlier of receipt by the Company of notice of
            and the Company otherwise becoming aware of any threatened or
            instituted claim, suit, petition or other proceedings with respect
            to the involuntary liquidation, dissolution or winding-up of the
            Company or to effect any other distribution of the assets of the
            Company among its shareholders for the purpose of winding up its
            affairs;

      (c)   immediately, upon receipt by the Company of a Retraction Request;

      (d)   immediately upon notice of redemption being given to holders of the
            Company Special Shares, upon the determination of a Redemption Date
            in accordance with the Share Provisions; and

      (e)   immediately upon the issuance by the Company of any Company Special
            Shares or rights to acquire Company Special Shares (other than the
            issuance of Company Special Shares and rights to acquire Company
            Special Shares in exchange for Company Common Shares pursuant to the
            Arrangement).

2.5 Delivery of Abgenix Common Shares to the Company and Abgenix Canada

      In furtherance of its obligations under Section 2.1(1)(d) and Section
2.1(1)(e), upon notice from the Company or Abgenix Canada of any event that
requires the Company or Abgenix Canada to cause to be delivered Abgenix Common
Shares to any holder of Company Special Shares, Abgenix shall forthwith deliver
to the relevant holder of Company Special Shares as directed by the Company or
Abgenix Canada the requisite number of Abgenix Common Shares to be received by,
and delivered to or to the order of, the former holder of the surrendered
Company Special Shares. All such Abgenix Common Shares shall be duly authorized
and validly issued as fully paid and shall be free and clear of any lien, claim
or encumbrance. In consideration of the issuance of each such Abgenix Common
Share, the Company or Abgenix Canada, as the case may be, shall subscribe a cash
amount equal to such amount as Abgenix and the Company or Abgenix Canada, as the
case may be, shall agree and shall be lawful.

2.6 Qualification of Abgenix Common Shares

      If any Abgenix Common Shares (or other shares or securities into which
Abgenix Common Shares may be reclassified or changed as contemplated by Section
2.7) to be delivered hereunder requires registration or qualification with or
approval of or the filing of any document, including any prospectus or similar
document or the taking of any proceeding with or the obtaining of any order,
ruling or consent from any governmental or regulatory authority under any United
States or Canadian federal, provincial, territorial or state securities or other
law or


                                       4
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regulation or pursuant to the rules and regulations of any securities or other
regulatory authority or the fulfillment of any other United States or Canadian
legal requirement before such shares (or such other shares or securities) may be
issued by Abgenix at the direction of Abgenix Canada or the Company, if
applicable, to the holder of surrendered Company Special Shares or in order that
such shares (or such other shares or securities) may be freely traded thereafter
(other than any restrictions of general application on transfer by reason of a
holder being a "control person" for purposes of Canadian securities law or the
equivalent thereof under applicable United States laws or stock exchange or
regulatory requirements), Abgenix will in good faith expeditiously take all such
actions and do all such things as are reasonably necessary to cause such Abgenix
Common Shares (or such other shares or securities) to be and remain duly
registered, qualified or approved under United States or Canadian law, as the
case may be but has no such obligations in respect of foreign laws; provided,
however, that (i) nothing in this Agreement, the Voting, Exchange and Cash Put
Trust Agreement, or the Plan of Arrangement shall be construed to impose any
obligation on Abgenix with respect to the filing and maintenance of a
registration statement with the United States Securities Exchange Commission in
addition to the provisions set forth in Section 2.4(d) of the Acquisition
Agreement, (ii) Abgenix shall be entitled to the right to suspend the use of the
Registration Statement (as defined therein) pursuant to Section 2.4(d) of the
Acquisition Agreement, and (iii) Section 2.4(d) of the Acquisition Agreement is
incorporated herein and made a part hereof notwithstanding any expiration of the
Acquisition Agreement. Abgenix may require evidence satisfactory to it, acting
reasonably, to be provided by a holder of Company Special Shares that no
prospectus or other disclosure document is required to be prepared or filed
under such foreign laws. If a holder fails to provide such evidence, Abgenix
Canada shall be entitled to issue cash equal to the Current Market Price of all
Abgenix Common Shares otherwise deliverable to such holder. Abgenix will in good
faith expeditiously take all such actions and do all such things as are
reasonably necessary or desirable to cause all Abgenix Common Shares (or such
other shares or securities) to be delivered hereunder to be listed, quoted or
posted for trading on all stock exchanges and quotation systems on which
outstanding Abgenix Common Shares (or such other shares or securities) have been
listed by Abgenix and remain listed and are quoted or posted for trading at such
time.

2.7 Economic Equivalence

      So long as any Company Special Shares not owned by Abgenix or its
Affiliates are outstanding:

      (a)   Abgenix will not, without prior approval of the Company and the
            prior approval of the holders of the Company Special Shares given in
            accordance with Section 10(2) of the Share Provisions:

            (i)   issue or distribute Abgenix Common Shares (or securities
                  exchangeable for or convertible into or carrying rights to
                  acquire Abgenix Common Shares) to the holders of all or
                  substantially all of the then outstanding Abgenix Common
                  Shares by way of stock dividend or other distribution, other
                  than an issue of Abgenix Common Shares (or securities
                  exchangeable for or convertible into or carrying rights to
                  acquire Abgenix Common Shares) to holders of Abgenix Common
                  Shares (i) who exercise an option to receive dividends in
                  Abgenix Common Shares (or securities


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<PAGE>

                  exchangeable for or convertible into or carrying rights to
                  acquire Abgenix Common Shares) in lieu of receiving cash
                  dividends, or (ii) pursuant to any dividend reinvestment plan
                  or scrip dividend;

            (ii)  issue or distribute rights, options or warrants to the holders
                  of all or substantially all of the then outstanding Abgenix
                  Common Shares entitling them to subscribe for or to purchase
                  Abgenix Common Shares (or securities exchangeable for or
                  convertible into or carrying rights to acquire Abgenix Common
                  Shares); or

            (iii) issue or distribute to the holders of all or substantially all
                  of the then outstanding Abgenix Common Shares, (A) securities
                  of Abgenix of any class other than Abgenix Common Shares
                  (other than shares convertible into or exchangeable for or
                  carrying rights to acquire Abgenix Common Shares); (B) rights,
                  options or warrants other than those referred to in Section
                  2.7(a)(ii); (C) evidences of indebtedness of Abgenix; or (D)
                  assets of Abgenix;

unless in each case the economic equivalent on a per share basis of such rights,
options, securities, shares, evidences of indebtedness or other assets is issued
or distributed simultaneously to holders of the Company Special Shares; provided
that, for greater certainty, the above restrictions shall not apply to any
securities issued or distributed by Abgenix in order to give effect to and to
consummate the transactions contemplated by, and in accordance with, the
Acquisition Agreement.

      (b)   Abgenix will not, without the prior approval of the Company and the
            prior approval of the holders of the Company Special Shares given in
            accordance with Section 10(2) of the Share Provisions:

            (i)   subdivide, redivide or change the then outstanding Abgenix
                  Common Shares into a greater number of Abgenix Common Shares;
                  or

            (ii)  reduce, combine, consolidate or change the then outstanding
                  Abgenix Common Shares into a lesser number of Abgenix Common
                  Shares; or

            (iii) reclassify or otherwise change Abgenix Common Shares or effect
                  an amalgamation, merger, reorganization or other transaction
                  affecting Abgenix Common Shares;

unless the same or an economically equivalent change shall simultaneously be
made to, or in the rights of the holders of, the Company Special Shares.

      (c)   Abgenix will ensure that the record date for any event referred to
            in Section 2.7(a) or Section 2.7(b), or (if no record date is
            applicable for such event) the effective date for any such event, is
            not less than five (5) Business Days after the date on which such
            event is declared or announced by Abgenix (with contemporaneous
            notification thereof by Abgenix to the Company).


                                       6
<PAGE>

      (d)   The Board of Directors of the Company shall determine, in good faith
            and in its sole discretion, economic equivalence for the purposes of
            any event referred to in Section 2.7(a) or Section 2.7(b) and each
            such determination shall be conclusive. In making each such
            determination, the following factors shall, without excluding other
            factors determined by the Board of Directors of the Company to be
            relevant, be considered by the Board of Directors of the Company:

            (i)   in the case of any stock dividend or other distribution
                  payable in Abgenix Common Shares, the number of such shares
                  issued in proportion to the number of Abgenix Common Shares
                  previously outstanding;

            (ii)  in the case of the issuance or distribution of any rights,
                  options or warrants to subscribe for or purchase Abgenix
                  Common Shares (or securities exchangeable for or convertible
                  into or carrying rights to acquire Abgenix Common Shares), the
                  relationship between the exercise price of each such right,
                  option or warrant and the Current Market Price of an Abgenix
                  Common Share;

            (iii) in the case of the issuance or distribution of any other form
                  of property (including without limitation any shares or
                  securities of Abgenix of any class other than Abgenix Common
                  Shares, any rights, options or warrants other than those
                  referred to in Section 2.7(d)(ii), any evidences of
                  indebtedness of Abgenix or any assets of Abgenix), the
                  relationship between the fair market value (as determined by
                  the Board of Directors of Abgenix in the manner above
                  contemplated) of such property to be issued or distributed
                  with respect to each outstanding Abgenix Common Share and the
                  Current Market Price of an Abgenix Common Share;

            (iv)  in the case of any subdivision, redivision or change of the
                  then outstanding Abgenix Common Shares into a greater number
                  of Abgenix Common Shares or the reduction, combination,
                  consolidation or change of the then outstanding Abgenix Common
                  Shares into a lesser number of Abgenix Common Shares or any
                  amalgamation, merger, reorganization or other transaction
                  affecting Abgenix Common Shares, the effect thereof upon the
                  then outstanding Abgenix Common Shares; and

            (v)   in all such cases, the general taxation consequences of the
                  relevant event to holders of Company Special Shares to the
                  extent that such consequences may differ from the taxation
                  consequences to holders of Abgenix Common Shares as a result
                  of differences between taxation laws of Canada and the United
                  States (except for any differing consequences arising as a
                  result of differing marginal taxation rates and without regard
                  to the individual circumstances of holders of Company Special
                  Shares).

The Company agrees that, to the extent required, upon due notice from Abgenix,
the Company will use its best efforts to take or cause to be taken such steps as
may be necessary for the purposes of ensuring that appropriate dividends are
paid or other distributions are made by the


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Company, or subdivisions, redivisions or changes are made to the Company Special
Shares, in order to implement the required economic equivalence with respect to
the Abgenix Common Shares and Company Special Shares as provided for in this
Section 2.7.

2.8 Tender Offers

      In the event that a tender offer, share exchange offer, issuer bid,
take-over bid or similar transaction with respect to Abgenix Common Shares (an
"Offer") is proposed by Abgenix or is proposed to Abgenix or its shareholders
and is recommended by the Board of Directors of Abgenix, or is otherwise
effected or to be effected with the consent or approval of the Board of
Directors of Abgenix, and the Company Special Shares are not redeemed by the
Company or purchased by Abgenix Canada pursuant to the Redemption Call Right,
Abgenix will use its reasonable efforts expeditiously and in good faith to take
all such actions and do all such things as are necessary or desirable to enable
and permit holders of Company Special Shares (other than Abgenix and its
Affiliates) to participate in such Offer to the same extent and on an
economically equivalent basis as the holders of Abgenix Common Shares, without
discrimination. Without limiting the generality of the foregoing, Abgenix will
use its reasonable efforts expeditiously and in good faith to ensure that
holders of Company Special Shares may participate in each such Offer without
being required to retract Company Special Shares as against the Company (or, if
so required, to ensure that any such retraction, shall be effective only upon,
and shall be conditional upon, the closing of such Offer and only to the extent
necessary to tender or deposit to the Offer). Nothing herein shall affect the
rights of the Company to redeem (or Abgenix Canada to purchase pursuant to the
Redemption Call Right) Company Special Shares, as applicable, in the event of an
Abgenix Control Transaction.

2.9 Ownership of Outstanding Shares

      Without the prior approval of the Company and the prior approval of the
holders of the Company Special Shares given in accordance with Section 10(2) of
the Share Provisions, Abgenix covenants and agrees in favour of the Company
that, as long as any outstanding Company Special Shares are owned by any Person
other than Abgenix or any of its Affiliates, Abgenix will be and remain the
direct or indirect beneficial owner of all issued and outstanding voting shares
in the capital of the Company and Abgenix Canada.

2.10 Abgenix and Affiliates Not to Vote Company Special Shares

      Abgenix covenants and agrees that it will appoint and cause to be
appointed proxyholders with respect to all Company Special Shares held by it and
its Affiliates for the sole purpose of attending each meeting of holders of
Company Special Shares in order to be counted as part of the quorum for each
such meeting. Abgenix further covenants and agrees that it will not, and will
cause its Affiliates not to, exercise any voting rights which may be exercisable
by holders of Company Special Shares from time to time pursuant to the Share
Provisions or pursuant to the provisions of the BC Act (or any successor or
other corporate statute by which the Company may in the future be governed) with
respect to any Company Special Shares held by it or by its Affiliates in respect
of any matter considered at any meeting of holders of Company Special Shares.


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2.11 Ordinary Market Purchases

      For certainty, nothing contained in this Agreement, including without
limitation the obligations of Abgenix contained in Section 2.8, shall limit the
ability of Abgenix (or any of its subsidiaries including, without limitation,
Abgenix Canada or the Company) to make ordinary market purchases of Abgenix
Common Shares in accordance with applicable laws and regulatory or stock
exchange requirements.

                                    ARTICLE 3
                               ABGENIX SUCCESSORS

3.1 Certain Requirements in Respect of Combination, etc.

      Abgenix shall not consummate any transaction (whether by way of
reconstruction, reorganization, consolidation, arrangement, merger, transfer,
sale, lease or otherwise) whereby all or substantially all of its undertaking,
property and assets would become the property of any other Person (or, in the
case of a merger, of the continuing corporation resulting therefrom) unless, but
may do so if:

      (a)   such other Person or continuing corporation (the "Abgenix
            Successor") by operation of law, becomes, without more, bound by the
            terms and provisions of this Agreement or, if not so bound,
            executes, prior to or contemporaneously with the consummation of
            such transaction, an agreement supplemental hereto and such other
            instruments (if any) as are reasonably necessary or advisable to
            evidence the assumption by the Abgenix Successor of liability for
            all moneys payable and property deliverable hereunder and the
            covenant of such Abgenix Successor to pay and deliver or cause to be
            delivered the same and its agreement to observe and perform all the
            covenants and obligations of Abgenix under this Agreement; and

      (b)   such transaction shall be upon such terms and conditions as
            substantially to preserve and not to impair in any material respect
            any of the rights, duties, powers and authorities of the other
            parties hereunder or the holders of the Company Special Shares.

3.2 Vesting of Powers in Successor

      Whenever the conditions of Section 3.1 have been duly observed and
performed, the parties, if required by Section 3.1, shall execute and deliver
the supplemental agreement provided for in Section 3.1(a) and thereupon the
Abgenix Successor shall possess and from time to time may exercise each and
every right and power of Abgenix under this Agreement in the name of Abgenix or
otherwise and any act or proceeding by any provision of this Agreement required
to be done or performed by the Board of Directors of Abgenix or any officers of
Abgenix may be done and performed with like force and effect by the directors or
officers of such Abgenix Successor.


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3.3 Wholly-Owned Subsidiaries

      Nothing herein shall be construed as preventing the amalgamation or merger
of any wholly-owned direct or indirect subsidiary of Abgenix with or into
Abgenix or the winding-up, liquidation or dissolution of any wholly-owned direct
or indirect subsidiary of Abgenix or any other distribution of the assets of any
wholly-owned direct or indirect subsidiary of Abgenix among the shareholders of
such subsidiary for the purpose of winding up its affairs, provided that all of
the assets of such subsidiary are transferred to Abgenix or another wholly-owned
direct or indirect subsidiary of Abgenix and any such transactions are expressly
permitted by this Article 3.

                                    ARTICLE 4
                                     GENERAL

4.1 Term

      This Agreement shall come into force and be effective as of the date
hereof and shall terminate and be of no further force and effect at such time as
no Company Special Shares (or securities or rights convertible into or
exchangeable for or carrying rights to acquire Company Special Shares) are held
by any Person other than Abgenix and any of its Affiliates.

4.2 Changes in Capital of Abgenix and the Company

      At all times after the occurrence of any event contemplated pursuant to
Section 2.7 and Section 2.8 or otherwise, as a result of which either Abgenix
Common Shares or the Company Special Shares or both are in any way changed, this
Agreement shall forthwith be amended and modified as necessary in order that it
shall apply with full force and effect, mutatis mutandis, to all new securities
into which Abgenix Common Shares or the Company Special Shares or both are so
changed and the parties hereto shall execute and deliver an Agreement in writing
giving effect to and evidencing such necessary amendments and modifications.

4.3 Severability

      If any term or other provision of this Agreement is invalid, illegal or
incapable of being enforced by any rule or law, or public policy, all other
conditions and provisions of this Agreement shall nevertheless remain in full
force and effect so long as the economic or legal substance of the transactions
contemplated hereby is not affected in any manner materially adverse to any
party. Upon such determination that any term or other provision is invalid,
illegal or incapable of being enforced, the parties hereto shall negotiate in
good faith to modify this Agreement so as to effect the original intent of the
parties as closely as possible in an acceptable manner to the end that the
transactions contemplated hereby are fulfilled to the fullest extent possible.

4.4 Amendments, Modifications

This Agreement may not be amended or modified except (a) as otherwise provided
in this Agreement, or (b) by an agreement in writing executed by the Company,
Abgenix Canada and


                                       10
<PAGE>

Abgenix and approved by the holders of the Company Special Shares in accordance
with Section 10(2) of the Share Provisions.

No amendment or modification or waiver of any of the provisions of this
Agreement otherwise permitted hereunder shall be effective unless made in
writing and signed by all of the parties hereto.

4.5 Ministerial Amendments

      Notwithstanding the provisions of Section 4.4, the parties to this
Agreement may in writing at any time and from time to time, without the approval
of the holders of the Company Special Shares, amend or modify this Agreement for
the purposes of:

      (a)   adding to the covenants of any or all parties provided that the
            Board of Directors of each of the Company, Abgenix Canada and
            Abgenix shall be of the good faith opinion that such additions will
            not be prejudicial to the rights or interests of the holders of the
            Company Special Shares;

      (b)   making such amendments or modifications not inconsistent with this
            Agreement as may be necessary or desirable with respect to matters
            or questions which, in the good faith opinion of the Board of
            Directors of each of the Company, Abgenix Canada and Abgenix, it may
            be expedient to make, provided that each such Board of Directors
            shall be of the good faith opinion that such amendments or
            modifications will not be prejudicial to the rights or interests of
            the holders of the Company Special Shares; or

      (c)   making such changes or corrections which, on the advice of counsel
            to the Company, Abgenix Canada and Abgenix, are required for the
            purpose of curing or correcting any ambiguity or defect or
            inconsistent provision or clerical omission or mistake or manifest
            error, provided that the Boards of Directors of each of the Company,
            Abgenix Canada and Abgenix shall be of the good faith opinion that
            such changes or corrections will not be prejudicial to the rights or
            interests of the holders of the Company Special Shares.

4.6 Meeting to Consider Amendments

      The Company, at the request of Abgenix, shall call a meeting or meetings
of the holders of the Company Special Shares for the purpose of considering any
proposed amendment or modification requiring approval pursuant to Section 4.4.
Any such meeting or meetings shall be called and held in accordance with the
articles of the Company, the Share Provisions and all applicable laws.

4.7 Enurement

      This Agreement shall be binding upon and enure to the benefit of the
parties hereto and their respective successors and permitted assigns.


                                       11
<PAGE>

4.8 Notices to Parties

All notices and other communications between the parties to this Agreement shall
be in writing and shall be deemed to have been given if delivered personally or
by confirmed telecopy to the parties at the following addresses (or at such
other address for any such party as shall be specified in like notice):

            If to Abgenix or Abgenix Canada:

            Abgenix, Inc.
            7601 Dumbarton Circle
            Fremont, CA 94555

            Attention: Mr. Kurt Leutzinger
            cc: Daniel Hunt, Esq.
            Telephone No.: (510) 608-6500
            Telecopier No.: (510) 608-6511

            with a copy (which shall not constitute notice) to:

            Stikeman Elliott
            Suite 1700
            Park Place, 666 Burrard Street
            Vancouver, Canada
            V6C 2X8

            Attention: Jonathan S. Drance, Esq.
            Telephone No.: (604) 631-1361
            Telecopier No.: (604) 681-1825

            and a copy (which shall not constitute notice) to:

            O'Melveny & Myers LLP
            Embarcadero Center West
            275 Battery Street
            San Francisco, CA 94111-3305

            Attention: Peter T. Healy, Esq.
            Telephone No.: (415) 984-8833
            Telecopier No.: (415) 984-8701

Any notice or other communication given personally shall be deemed to have been
given and received upon delivery thereof and if given by telecopy shall be
deemed to have been given and received on the date of confirmed receipt thereof
unless such day is not a Business Day, in which case it shall be deemed to have
been given and received upon the immediately following Business Day.


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<PAGE>

4.9 Counterparts

      This Agreement may be executed in facsimile and in counterparts, each of
which shall be deemed an original, and all of which taken together shall
constitute one and the same instrument.

4.10 Jurisdiction

      This Agreement shall be construed and enforced in accordance with the laws
of the Province of British Columbia and the laws of Canada applicable therein.

4.11 Attornment

      Each of the parties hereto agrees that any action or proceeding arising
out of or relating to this Agreement may be instituted in the courts of the
Province of British Columbia and waives any objection which it may have now or
hereafter to the venue of any such action or proceeding.

      IN WITNESS WHEREOF, the parties hereto have caused this Agreement to be
duly executed as of the date first above written.

                                    ABGENIX, INC.

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

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

                                    Title:
                                           -----------------------------


                                    ABGENIX CANADA CORPORATION

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

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

                                    Title:
                                           -----------------------------


                                    IMMGENICS PHARMACEUTICALS INC.

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

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

                                    Title:
                                           -----------------------------


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