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                                                                     EXHIBIT 3.1

                                 FOURTH RESTATED
                         CERTIFICATE OF INCORPORATION OF
                                  TOLERRX, INC.

     TolerRx, Inc. (the "CORPORATION"), a corporation organized and existing
under and by virtue of the General Corporation Law of the State of Delaware (the
"GENERAL CORPORATION LAW"), hereby certifies as follows:

     FIRST:    The name of the Corporation is TolerRx, Inc. The Corporation was
formed on July 6, 2000, under the name Immagen, Inc. A Third Restated
Certificate of Incorporation (the "CERTIFICATE OF INCORPORATION") was filed with
the Secretary of State of Delaware on November 1, 2002.

     SECOND:   This Fourth Restated Certificate of Incorporation: (i) was duly
adopted in accordance with the provisions of Sections 242 and 245 of the General
Corporation Law; and (ii) was approved by written consent of a majority of the
stockholders of the Corporation given in accordance with the provisions of
Section 228 of the General Corporation Law.

     THIRD:    The text of the Certificate of Incorporation of the Corporation
is hereby further restated and amended to read in its entirety as follows:

                                    ARTICLE I
                                      NAME

     The name of the corporation (the "CORPORATION") is TolerRx, Inc.

                                   ARTICLE II
                                REGISTERED AGENT

     The address of the Corporation's registered office in the State of Delaware
is 1209 Orange Street, Corporation Trust Center, City of Wilmington, County of
New Castle, State of Delaware. The name of its registered agent is The
Corporation Trust Company.

                                   ARTICLE III
                                     PURPOSE

     The nature of the business or purposes to be conducted or promoted by the
Corporation is as follows:

     To engage in any lawful act or activity for which corporations may be
organized under the General Corporation Law of the State of Delaware.

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                                   ARTICLE IV
                                  CAPITAL STOCK

     The total number of shares of all classes of capital stock which the
Corporation shall have authority to issue is Fifty-Five Million (55,000,000)
shares, consisting solely of:

          -    Fifty Million (50,000,000) shares of common stock, par value
               $0.001 per share ("COMMON STOCK"); and

          -    Five Million (5,000,000) shares of preferred stock, par value
               $0.001 per share ("PREFERRED STOCK").

     The following is a statement of the powers, designations, preferences,
privileges, and relative rights in respect of each class of capital stock of the
Corporation.

     A.   COMMON STOCK.

     1.   GENERAL.     The voting, dividend and liquidation rights of the
holders of Common Stock are subject to and qualified by the rights of the
holders of Preferred Stock.

     2.   VOTING.      The holders of Common Stock are entitled to one vote for
each share held at all meetings of stockholders. There shall be no cumulative
voting.

     3.   DIVIDENDS.   Dividends may be declared and paid on the Common Stock
from funds lawfully available therefor if, as and when determined by the Board
of Directors and subject to any preferential dividend rights of any then
outstanding shares of Preferred Stock.

     4.   LIQUIDATION. Upon the dissolution or liquidation of the Corporation,
whether voluntary or involuntary, holders of Common Stock will be entitled to
receive all assets of the Corporation available for distribution to its
stockholders, subject to any preferential rights of any then outstanding shares
of Preferred Stock.

     B.   PREFERRED STOCK.

     Shares of Preferred Stock may be issued from time to time in one or more
series, each of such series to have such powers, designations, preferences, and
relative, participating, optional, or other special rights, if any, and such
qualifications and restrictions, if any, of such preferences and rights, as are
stated or expressed in the resolution or resolutions of the Board of Directors
providing for such series of Preferred Stock. Different series of Preferred
Stock shall not be construed to constitute different classes of shares for the
purposes of voting by classes unless expressly so provided in such resolution or
resolutions.

     Authority is hereby granted to the Board of Directors from time to time to
issue the Preferred Stock in one or more series, and in connection with the
creation of any such series, by resolution or resolutions to determine and fix
the powers, designations, preferences, and relative, participating, optional, or
other special rights, if any, and the qualifications and restrictions, if

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any, of such preferences and rights, including without limitation dividend
rights, conversion rights, voting rights (if any), redemption privileges, and
liquidation preferences, of such series of Preferred Stock (which need not be
uniform among series), all to the fullest extent now or hereafter permitted by
the General Corporation Law of the State of Delaware. Without limiting the
generality of the foregoing, the resolution or resolutions providing for the
creation or issuance of any series of Preferred Stock may provide that such
series shall be superior to, rank equally with, or be junior to the Preferred
Stock of any other series, all to the fullest extent permitted by law. No
resolution, vote, or consent of the holders of the capital stock of the
Corporation shall be required in connection with the creation or issuance of any
shares of any series of Preferred Stock authorized by and complying with the
conditions of this Fourth Restated Certificate of Incorporation, the right to
any such resolution, vote, or consent being expressly waived by all present and
future holders of the capital stock of the Corporation.

     Any resolution or resolutions adopted by the Board of Directors pursuant to
the authority vested in them by this Article IV shall be set forth in a
certificate of designation along with the number of shares of stock of such
series as to which the resolution or resolutions shall apply and such
certificate shall be executed, acknowledged, filed, recorded, and shall become
effective, in accordance with Section 103 of the General Corporation Law of the
State of Delaware. Unless otherwise provided in any such resolution or
resolutions, the number of shares of stock of any such series to which such
resolution or resolutions apply may be increased (but not above the total number
of authorized shares of the class) or decreased (but not below the number of
shares thereof then outstanding) by a certificate likewise executed,
acknowledged, filed and recorded, setting forth a statement that a specified
increase or decrease therein has been authorized and directed by a resolution or
resolutions likewise adopted by the Board of Directors. In case the number of
such shares shall be decreased, the number of shares so specified in the
certificate shall resume the status which they had prior to the adoption of the
first resolution or resolutions. When no shares of any such class or series are
outstanding, either because none were issued or because none remain outstanding,
a certificate setting forth a resolution or resolutions adopted by the Board of
Directors that none of the authorized shares of such class or series are
outstanding, and that none will be issued subject to the certificate of
designations previously filed with respect to such class or series, may be
executed, acknowledged, filed and recorded in the same manner as previously
described and it shall have the effect of eliminating from this Fourth Restated
Certificate of Incorporation all matters set forth in the certificate of
designations with respect to such class or series of stock. If no shares of any
such class or series established by a resolution or resolutions adopted by the
Board of Directors have been issued, the voting powers, designations,
preferences and relative, participating, optional or other rights, if any, with
the qualifications, limitations or restrictions thereof, may be amended by a
resolution or resolutions adopted by the Board of Directors. In the event of any
such amendment, a certificate which (i) states that no shares of such class or
series have been issued, (ii) sets forth the copy of the amending resolution or
resolutions and (iii) if the designation of such class or series is being
changed, indicates the original designation and the new designation, shall be
executed, acknowledged, filed, recorded, and shall become effective, in
accordance with Section 103 of the General Corporation Law of the State of
Delaware.

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                                    ARTICLE V
                               BOARD OF DIRECTORS

     The following provisions are inserted for the management of the business
and for the conduct of the affairs of the Corporation and for defining and
regulating the powers of the Corporation and its directors and stockholders and
are in furtherance and not in limitation of the powers conferred upon the
Corporation by statute:

          (a)  The number of directors constituting the full Board of Directors
     shall be as determined by the Board of Directors from time to time. Members
     of the Board of Directors shall hold office until the annual meeting of the
     Corporation's stockholders at which their respective successors are elected
     and qualified or until their earlier death, incapacity resignation or
     removal. Members of the Board of Directors may be removed only for cause
     and only by the vote of the holders of a majority of the shares of the
     Corporation's stock entitled to vote for the election of directors. Except
     as the General Corporation Law may otherwise require, in the interim
     between annual meetings of stockholders or special meetings of stockholders
     called for the election of directors and/or for the removal of one or more
     directors and for the filling of any vacancy in that connection, any
     vacancies in the Board of Directors, including unfilled vacancies resulting
     from the removal of directors for cause, may be filled by the vote of a
     majority of the remaining directors then in office, although less than a
     quorum, or by the sole remaining director.

          (b)  The Board of Directors shall have the power and authority: (i) to
     adopt, amend or repeal By-Laws of the Corporation, subject only to such
     limitations, if any, as may be from time to time imposed by other
     provisions of this Certificate, by law, or by the By-Laws; and (ii) to the
     full extent permitted or not prohibited by law, and without the consent of
     or other action by the stockholders, to authorize or create mortgages,
     pledges or other liens or encumbrances upon any or all of the assets, real,
     personal or mixed, and franchises of the Corporation, including
     after-acquired property, and to exercise all of the powers of the
     Corporation in connection therewith.

                                   ARTICLE VI
                             LIMITATION OF LIABILITY

     No director of the Corporation shall be personally liable to the
Corporation or to any of its stockholders for monetary damages for breach of
fiduciary duty as a director, notwithstanding any provision of law imposing such
liability; provided, however, that to the extent required from time to time by
applicable law, this Article VI shall not eliminate or limit the liability of a
director, to the extent such liability is provided by applicable law, (i) for
any breach of the director's duty of loyalty to the corporation or its
stockholders, (ii) for acts or omissions not in good faith or which involve
intentional misconduct or a knowing violation of law, (iii) under Section 174 of
Title 8 of the Delaware Code, or (iv) for any transactions from which the
director derived an improper personal benefit. No

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amendment to or repeal of this Article VI shall apply to or have any effect on
the liability or alleged liability of any director for or with respect to any
acts or omissions of such director occurring prior to the effective date of such
amendment or repeal.

                                   ARTICLE VII
                                 INDEMNIFICATION

     The Corporation shall, to the fullest extent permitted by Section 145 of
the General Corporation Law of Delaware, as amended from time to time, indemnify
each person who was or is a party or is threatened to be made a party to any
threatened, pending or completed action, suit or proceeding, whether civil,
criminal, administrative or investigative, by reason of the fact that he is or
was, or has agreed to become, a director or officer of the Corporation, or is or
was serving, or has agreed to serve, at the request of the Corporation, as a
director, officer or trustee of, or in a similar capacity with, another
corporation, partnership, joint venture, trust or other enterprise (including
any employee benefit plan), or by reason of any action alleged to have been
taken or omitted in such capacity, against all expenses (including attorneys'
fees), judgements, fines and amounts paid in settlement actually and reasonably
incurred by him or on his behalf in connection with such action, suit or
proceeding and any appeal therefrom, and as further provided in the By-Laws.

                                  ARTICLE VIII
                          COMPROMISES AND ARRANGEMENTS

     Whenever a compromise or arrangement is proposed between the Corporation
and its creditors or any class of them and/or between the Corporation and its
stockholders or any class of them, any court of equitable jurisdiction within
the State of Delaware may, on the application in a summary way of the
Corporation or of any creditor or stockholder thereof or on the application of
any receiver or receivers appointed for the Corporation under the provisions of
Section 391 of Title 8 of the Delaware Code or on the application of trustees in
dissolution or of any receiver or receivers appointed for the Corporation under
the provisions of Section 279 of Title 8 of the Delaware Code, order a meeting
of the creditors or class of creditors, and/or of the stockholders or class of
stockholders of the Corporation, as the case may be, to be summoned in such a
manner as the said court directs. If a majority of the number representing
three-fourths (3/4ths) in value of the creditors or class of creditors, and/or
of the stockholders or class of stockholders of the Corporation, as the case may
be, agree to any compromise or arrangement and to any reorganization of the
Corporation as a consequence of such compromise or arrangement, the compromise
or arrangement and the said reorganization shall, if sanctioned by the court to
which the said application has been made, be binding on all creditors or class
of creditors, and/or stockholders or class of stockholders of the Corporation,
as the case may be, and also on the Corporation.

                                   ARTICLE IX
                               STOCKHOLDER ACTION

     Any action required or permitted to be taken by the stockholders of the
Corporation may

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be taken only at a duly called annual or special meeting of the stockholders,
and not by written consent in lieu of such a meeting, and special meetings of
stockholders may be called only by the Chairman of the Board of Directors, the
President, or a majority of the Board of Directors.

                                    ARTICLE X
                     BOARD DISCRETION IN CONSIDERING OFFERS

     The Board of Directors, when considering a tender offer or merger or
acquisition proposal, may take into account factors in addition to potential
economic benefits to stockholders, including without limitation (i) comparison
of the proposed consideration to be received by stockholders in relation to the
then current market price of the Corporation's capital stock, the estimated
current value of the Corporation in a freely negotiated transaction, and the
estimated future value of the Corporation as an independent entity and (ii) the
impact of such a transaction on the employees, suppliers, and customers of the
Corporation and its effect on the communities in which the Corporation operates.

                                   ARTICLE XI
                                   AMENDMENTS

     The affirmative vote of the holders of at least 67% of the outstanding
voting stock of the Corporation (in addition to any separate class vote that may
in the future be required pursuant to the terms of any outstanding Preferred
Stock) shall be required to amend or repeal the provisions of Articles IV (to
the extent it relates to the authority of the Board of Directors to issue shares
of Preferred Stock in one or more series, the terms of which may be determined
by the Board of Directors), V, VII, IX or XI of this Amended and Restated
Certificate of Incorporation or to reduce the numbers of authorized shares of
Common Stock or Preferred Stock.

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     IN WITNESS WHEREOF, the Corporation has caused this Fourth Restated
Certificate of Incorporation to be duly executed by its President this ___ day
of ______________, 2003.

                                         TOLERRX, INC.


                                         By:
                                            ----------------------------------
                                             Douglas J. Ringler
                                             Chief Executive Officer

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