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                                                                    EXHIBIT 99.1

                      [AVANT IMMUNOTHERAPEUTICS, INC. LOGO]


FOR IMMEDIATE RELEASE/FEBRUARY 13, 2004


<Table>
<S>                              <C>                               <C>
Una S. Ryan, Ph.D.               Avery W. Catlin                   FOR MEDIA:
President and CEO                Chief Financial Officer           Joan Kureczka/Jesse Fisher
AVANT Immunotherapeutics, Inc.   AVANT Immunotherapeutics, Inc.    J. Kureczka Associates
(781) 433-0771                   (781) 433-0771                    (415) 821-2413
                                 info@avantimmune.com              jkureczka@aol.com
</Table>


                  AVANT ANNOUNCES $25 MILLION EQUITY FINANCING

NEEDHAM, MA (FEBRUARY 13, 2004): AVANT Immunotherapeutics, Inc. (Nasdaq: AVAN)
announced today that it has received commitments from institutional investors
for 8,965,000 shares of its common stock at a price of $2.75 per share for gross
proceeds of approximately $25 million. No warrants were issued in connection
with the offering. The shares were offered directly to selected institutional
investors under a shelf registration statement filed by AVANT in November 2003
with the Securities and Exchange Commission.

"This financing provides the resources to fund our clinical development
programs, including TP10 for cardiac surgery, provides for our general working
capital needs and enables us to pursue licensing and acquisition opportunities
for complementary technologies and products," said Una S. Ryan, Ph.D., President
and Chief Executive Officer of AVANT Immunotherapeutics. "Further, our cash
balances now exceed our currently anticipated requirements for the next two
years."

AVANT Immunotherapeutics, Inc. discovers, develops and sells innovative vaccines
and therapeutics that harness the human immune system to prevent and treat
disease. The company has developed a broad, well-staged pipeline of vaccines and
therapeutics for large, high-value, under-served markets. Six of AVANT's
products are in clinical development. These include an oral human rotavirus
vaccine, a treatment to reduce complement-mediated tissue damage associated with
cardiac by-pass surgery, and a novel vaccine for cholesterol management. AVANT
has also assembled a technology platform that enables the creation of
rapid-protecting, single-dose, oral vaccines that remain stable without
refrigeration. The company is


                                    - more -

119 FOURTH AVENUE  NEEDHAM, MA 02494-2725 USA  781-433-0771  FAX 781-433-0262
www.avantimmune.com

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AVANT ANNOUNCES $25 MILLION EQUITY FINANCING
FEBRUARY 13, 2004
Page 2

developing applications of this vaccine technology in four areas: biodefense,
travelers' vaccines, global health needs, and human food safety. AVANT's goal is
to demonstrate proof-of-concept for its products before leveraging further
development through both traditional pharmaceutical partnerships and
collaborations with governmental and other organizations.

Additional information on AVANT Immunotherapeutics, Inc. can be obtained through
our site on the World Wide Web: http://www.avantimmune.com.

THE OFFERING HAS BEEN MADE BY MEANS OF A PROSPECTUS THAT MAY BE OBTAINED FROM
AVERY W. CATLIN, CHIEF FINANCIAL OFFICER OF THE COMPANY. THIS PRESS RELEASE
SHALL NOT CONSTITUTE AN OFFER TO SELL OR THE SOLICITATION OF AN OFFER TO BUY,
NOR SHALL THERE BE ANY SALE OF THESE SECURITIES IN ANY STATE IN WHICH SUCH
OFFER, SOLICITATION OR SALE WOULD BE UNLAWFUL PRIOR TO REGISTRATION OR
QUALIFICATION UNDER SECURITIES LAWS OF ANY SUCH STATE.

SAFE HARBOR STATEMENT UNDER THE PRIVATE SECURITIES LITIGATION REFORM ACT OF
1995: THIS RELEASE INCLUDES FORWARD-LOOKING STATEMENTS WHICH REFLECT AVANT'S
CURRENT VIEWS WITH RESPECT TO FUTURE EVENTS AND FINANCIAL PERFORMANCE. THESE
FORWARD-LOOKING STATEMENTS ARE BASED ON MANAGEMENT'S BELIEFS AND ASSUMPTIONS AND
INFORMATION CURRENTLY AVAILABLE. THE WORDS "BELIEVE", "EXPECT", "ANTICIPATE",
"INTEND", "ESTIMATE", "PROJECT" AND SIMILAR EXPRESSIONS WHICH DO NOT RELATE
SOLELY TO HISTORICAL MATTERS IDENTIFY FORWARD-LOOKING STATEMENTS. INVESTORS
SHOULD BE CAUTIOUS IN RELYING ON FORWARD-LOOKING STATEMENTS BECAUSE THEY ARE
SUBJECT TO A VARIETY OF RISKS, UNCERTAINTIES, AND OTHER FACTORS THAT COULD CAUSE
ACTUAL RESULTS TO DIFFER MATERIALLY FROM THOSE EXPRESSED IN ANY SUCH
FORWARD-LOOKING STATEMENTS. THESE FACTORS INCLUDE, BUT ARE NOT LIMITED TO: (1)
THE INTEGRATION OF AVANT'S ACQUIRED UPT TECHNOLOGY AND PROGRAMS WITH ITS ALREADY
EXISTING TECHNOLOGY AND PROGRAMS; (2) THE ABILITY TO ADAPT AVANT'S VECTORING
SYSTEMS TO DEVELOP NEW, SAFE AND EFFECTIVE ORALLY ADMINISTERED VACCINES AGAINST
ANTHRAX AND PLAGUE OR OTHER BIOTERRORISM THREATS OR ANY OTHER MICROBES USED AS
BIOWEAPONS; (3) THE ABILITY TO SUCCESSFULLY COMPLETE DEVELOPMENT AND
COMMERCIALIZATION OF CHOLERAGARDE(TM) (PERU-15), TP10, TY800, CETI-1 AND OF
OTHER PRODUCTS; (4) THE COST, TIMING, SCOPE AND RESULTS OF ONGOING SAFETY AND
EFFICACY TRIALS OF CHOLERAGARDE(TM) (PERU-15), TP10, TY800, CETI-1 AND OTHER
PRECLINICAL AND CLINICAL TESTING; (5) THE ABILITY TO SUCCESSFULLY COMPLETE
PRODUCT RESEARCH AND FURTHER DEVELOPMENT, INCLUDING ANIMAL, PRE-CLINICAL AND
CLINICAL STUDIES OF CHOLERAGARDE(TM) (PERU-15), TP10, TY800, CETI-1 AND OTHER
PRODUCTS; (6) THE ABILITY TO MANAGE MULTIPLE LATE STAGE CLINICAL TRIALS FOR A
VARIETY OF PRODUCT CANDIDATES; (7) THE VOLUME AND PROFITABILITY OF PRODUCT SALES
OF MEGAN(R)VAC 1 AND OTHER FUTURE PRODUCTS; (8) CHANGES IN EXISTING AND
POTENTIAL RELATIONSHIPS WITH CORPORATE COLLABORATORS; (9) THE COST, DELIVERY AND
QUALITY OF CLINICAL AND COMMERCIAL GRADE MATERIALS SUPPLIED BY CONTRACT
MANUFACTURERS; (10) THE TIMING, COST AND UNCERTAINTY OF OBTAINING REGULATORY
APPROVALS TO USE CHOLERAGARDE(TM) (PERU-15) AND TY800, AMONG OTHER PURPOSES, TO
PROTECT TRAVELERS AND PEOPLE IN ENDEMIC REGIONS FROM DIARRHEA CAUSING DISEASES,
TO USE CETI-1, AMONG OTHER PURPOSES, TO RAISE SERUM HDL CHOLESTEROL LEVELS, TO
USE TP10 TO IMPROVE PATIENT OUTCOMES FOLLOWING CARDIAC BY-PASS SURGERY, AND FOR
OTHER PRODUCTS; (11) THE COST AND TIMING OF AVANT'S CONSTRUCTION, BUILD-OUT AND
VALIDATION OF A FALL RIVER PILOT MANUFACTURING FACILITY; (12) THE ABILITY TO
OBTAIN SUBSTANTIAL ADDITIONAL FUNDING; (13) THE ABILITY TO DEVELOP AND
COMMERCIALIZE PRODUCTS BEFORE COMPETITORS; (14) THE ABILITY TO RETAIN CERTAIN
MEMBERS OF MANAGEMENT; AND (15) OTHER FACTORS DETAILED FROM TIME TO TIME IN
FILINGS WITH THE SECURITIES AND EXCHANGE COMMISSION. WE EXPRESSLY DISCLAIM ANY
RESPONSIBILITY TO UPDATE FORWARD-LOOKING STATEMENTS.

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