Disclosed is a class of compounds referred to herein as effector compounds. Effector compounds are useful in connection with the modulation of an immune response. Modulation refers to the ability of the effector compounds of the present invention to either enhance (antigen supercharging) or inhibit (immunosuppressant activities) antigen presentation, depending upon the nature of the particular effector compound and the therapeutic context. Effector compounds include peptides, modified peptides and peptidomimetics. Also disclosed are methods for modulating presentation of an MHC class II restricted antigenic peptide to a T cell. Also disclosed are effector compounds demonstrated to act specifically on a human MHC class II allele. Also disclosed is a second class of compounds, referred to herein as immunomodulatory organic compounds. Such compounds are identified by a method which includes the following steps: providing a first complex comprising an MHC class II molecule to which an antigenic peptide has been bound; contacting the first complex with mammalian Ii key peptide LRMKLPKPPKPVSKMR (SEQ ID NO:1) (or modifications thereof including peptidomimetics), thereby forming a second complex; and screening organic molecules for compounds which bind to the second complex but not to the first complex, and which exhibit immunomodulatory activity. Compounds identified in this manner can be used to modulate an immune response in a mammal.