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                                                                     EXHIBIT 5.1


                               PALMER & DODGE LLP
                    ONE BEACON STREET, BOSTON, MA 02108-3190


TELEPHONE: (617) 573-0100                              FACSIMILE: (617) 227-4420

                                             September 17, 2001


Antigenics Inc.
630 Fifth Street, Suite 2100
New York, New York 10111


Ladies and Gentlemen:

         We are rendering this opinion in connection with the Registration
Statement on Form S-8 (the "Registration Statement") filed by Antigenics Inc.
(the "Company") with the Securities and Exchange Commission under the Securities
Act of 1933, as amended, on or about the date hereof. The Registration Statement
relates to the registration of 172,311 shares (the "Shares") of the Company's
common stock, $.01 par value, issuable upon exercise of certain options held by
Aronex Pharmaceuticals, Inc. ("Aronex") employees and directors as provided for
under the terms of the agreement (the "Agreement") by which the Company acquired
Aronex.

         We have acted as your counsel in connection with the preparation of the
Registration Statement and are familiar with the proceedings taken by the
Company in connection with the authorization and issuance of the Shares. We have
examined all such documents as we consider necessary to enable us to render this
opinion.

         Based upon the foregoing, we are of the opinion that the Shares have
been duly authorized for issuance and, when issued in accordance with the terms
of the Agreement, will be legally issued, fully paid and nonassessable.

         The foregoing opinion is limited to Delaware General Corporation Law,
including the applicable provisions of the Delaware Constitution and reported
judicial decisions interpreting these laws, and the federal laws of the United
States.

         We hereby consent to the filing of this opinion as an exhibit to the
Registration Statement.

                                          Very truly yours,


                                          /s/ Palmer & Dodge LLP
                                          -------------------------------
                                          PALMER & DODGE LLP