SECURITIES AND EXCHANGE COMMISSION
                            Washington, D.C. 20549

                                 SCHEDULE 13G


                                 (Rule 13d-102)


             INFORMATION STATEMENT PURSUANT TO RULES 13d-1 AND 13d-2


                   Under the Securities Exchange Act of 1934
                          (Amendment No. 1)

                             IDEC PHARMACEUTICALS CORP / DE
             -----------------------------------------------------
                                (Name of Issuer)

                                  Common Stock
              -----------------------------------------------------
                         (Title of Class of Securities)

                                    449370105
             -----------------------------------------------------
                                 (CUSIP Number)



Check the following box if a fee is being paid with this statement [ ].


----------
*The remainder of this cover page shall be filled out for a reporting person's
initial filing on this form with respect to the subject class of securities, and
for any subsequent amendment containing information which would alter the
disclosures provided in a prior cover page.

The information required in the remainder of this cover page shall not be deemed
to be "filed" for the purpose of Section 18 of the Securities Exchange Act of
1934 ("Act") or otherwise subject to the liabilities of that section of the Act
but shall be subject to all other provisions of the Act (however, see the
Notes).



CUSIP No. 449370105                    13G                    Page 2  of 6 Pages



--------------------------------------------------------------------------------
   1.  NAME OF REPORTING PERSON(S)
       S.S. OR I.R.S. IDENTIFICATION NO. OF ABOVE PERSON(S)

       Morgan  Stanley Dean  Witter &  Co.
       IRS  #  39-314-5972
--------------------------------------------------------------------------------
   2.  CHECK THE APPROPRIATE BOX IF A MEMBER OF A GROUP*
                                                             (a)  [  ]
                                                             (b)  [  ]
--------------------------------------------------------------------------------
   3.  SEC USE ONLY


--------------------------------------------------------------------------------
   4.  CITIZENSHIP OR PLACE OF ORGANIZATION

       The state of organization is Delaware.
--------------------------------------------------------------------------------
   NUMBER OF      5.   SOLE VOTING POWER
     SHARES            0
  BENEFICIALLY    --------------------------------------------------------------
    OWNED BY      6.   SHARED VOTING POWER
      EACH             4,780,692
   REPORTING      --------------------------------------------------------------
     PERSON       7.   SOLE DISPOSITIVE POWER
      WITH             0
                  --------------------------------------------------------------
                  8.   SHARED DISPOSITIVE POWER
                       7,845,222
--------------------------------------------------------------------------------
   9.  AGGREGATE AMOUNT BENEFICIALLY OWNED BY EACH REPORTING PERSON

      7,845,222
--------------------------------------------------------------------------------
  10.  CHECK BOX IF THE AGGREGATE AMOUNT IN ROW (9) EXCLUDES CERTAIN SHARES*


--------------------------------------------------------------------------------
  11.  PERCENT OF CLASS REPRESENTED BY AMOUNT IN ROW (9)

       5.14%
--------------------------------------------------------------------------------
  12.  TYPE OF REPORTING PERSON*

       IA, CO
--------------------------------------------------------------------------------
                     *SEE INSTRUCTIONS BEFORE FILLING OUT!


CUSIP No. 449370105                   13G                   Page 3  of 6 Pages





Item 1.     (a)   Name of Issuer:
                  IDEC PHARMACEUTICALS CORP / DE
                 --------------------------------------------------------------
            (b)   Address of Issuer's Principal Executive Offices:
                  11011 TORREYANA ROAD
                  SAN DIEGO
                  CA 92121
                  --------------------------------------------------------------
Item 2.     (a)   Name of Person Filing:

                  Morgan Stanley Dean Witter & Co.
                  --------------------------------------------------------------
            (b)   Address of Principal Business Office, or if None, Residence:

                  1585 Broadway
                  New  York, New  York  10036
                  --------------------------------------------------------------
            (c)   Citizenship:

                  Incorporated by reference to Item 4 of the cover page
                  pertaining to each reporting person.

                  --------------------------------------------------------------
            (d)   Title of Class of Securities:
                  Common Stock
                  --------------------------------------------------------------
            (e)   CUSIP Number:
                  449370105
                  --------------------------------------------------------------

Item 3.          Morgan Stanley Dean Witter & Co. is a parent holding company.






CUSIP No. 449370105                   13-G                   Page 4  of  6 Pages



Item 4.     Ownership.

            Incorporated by reference to Items (5) - (9) and (11) of the
            cover page.

Item 5.     Ownership of Five Percent or Less of a Class.

            Inapplicable.

Item 6.     Ownership of More Than Five Percent on Behalf of Another Person.

            Accounts managed on a discretionary basis by Morgan Stanley Dean
            Witter & Co.  are known to have the right to receive or the power
            to direct the receipt of dividends from, or the proceeds from the
            sale of  such  securities.  No such account holds more than 5
            percent of the class.

Item 7.     Identification and Classification of the Subsidiary which Acquired
            the Security Being Reported on By the Parent Holding Company.

Item 8.     Identification and Classification of Members of the Group.

Item 9.     Notice of Dissolution of Group.

Item 10.    Certification.

            By signing below I certify that, to the best of my knowledge and
            belief, the securities referred to above were acquired in the
            ordinary course of business and were not acquired for the purpose
            of and do not have the effect of changing or influencing the
            control of the issuer of such securities and were not acquired in
            connection with or as a participant in any transaction having
            such purpose or effect.


CUSIP No. 449370105                    13-G                   Page 5  of 6 Pages


                                  Signature.


            After reasonable inquiry and to the best of my knowledge and
            belief, I certify that the information set forth in this
            statement is true, complete and correct.


Date:       February 12, 2002

Signature:  /s/ Dennine Bullard
            -----------------------------------------------------------------

Name/Title  Dennine Bullard /Vice President Morgan Stanley & Co. Incorporated
            -----------------------------------------------------------------
            MORGAN STANLEY DEAN WITTER & CO.



                       INDEX TO EXHIBITS                           PAGE
                       -----------------                           ----


EXHIBIT 2       Secretary's Certificate Authorizing Dennine Bullard    6
                to Sign on behalf of Morgan Stanley Dean Witter & Co.





* Attention. Intentional misstatements or omissions of fact constitute federal
  criminal violations (see 18 U.S.C. 1001).

(022597DTI)

                   EX-99
            SECRETARY'S CERTIFICATE





                                                             Page 6 of 6 Pages
                                  EXHIBIT 1                   ------------------

                 MORGAN STANLEY, DEAN WITTER, DISCOVER & CO.

                           SECRETARY'S CERTIFICATE



         I, Charlene R. Herzer, a duly elected and Assistant Secretary
         of Morgan Stanley Dean Witter & Co., a corporation organized and
         existing under the laws of the State of Delaware (the
         "Corporation"), certify that Stuart J.M. Breslow, Robert G. Koppenol
         and Dennine Bullard are authorized to sign reports to be filed under
         Sections 13 and 16 of the Securities Exchange Act of 1934 (the "Act")
         pursuant to the following:

             (1)      On May 31, 1997, the Board of Directors of the
                      Corporation granted authority to Stuart J.M.
                      Breslow and Robert G. Koppenol to sign reports to be
                      filed under Sections 13 and 16 of the Act on behalf of
                      the Corporation.

             (2)      On February 23, 2000, Donald G. Kempf, Jr., the Executive
                      Vice President, Chief Legal Officer and Secretary of the
                      Corporation, delegated authority to Dennine Bullard to
                      sign reports to be filed under Sections 13 and 16 of the
                      Act on behalf of the Corporation.  Pursuant to Section
                      7.01 of the Bylaws of the Corporation and resolutions
                      approved by the Board of Directors of the Corporation on
                      September 25, 1998, the Chief Legal Officer is authorized
                      to enter into agreements and other instruments on behalf
                      of the Corporation and may delegate such powers to others
                      under his jurisdiction.

          IN WITNESS WHEREOF, I have hereunto set my name and affixed the seal
          of the Corporation as of the 1st day of March, 2000.

                                                ______________________________
                                                Charlene R. Herzer
                                                Assistant Secretary