Abstract: Solution-based methods for identifying an MHC-binding peptide or measuring affinity of MHC-binding peptides for an MHC monomer, or modified MHC monomer by incubating at least one MHC monomer or modified MHC monomer having a bound template MHC-binding peptide, an excess amount of a competitor peptide, and a tracer MHC-binding peptide tagged with a detectable label so as to allow competition binding between the three peptides At least a portion of the competitor peptide exchanges with the template peptide and a difference in signal produced by the detectable label in the total sample as compared with signal produced solely by monomers after the competition assay is obtained and used to calculate affinity of the competitor peptide for the monomer. These methods are useful in peptide discovery programs and exchanged monomers can be further tested for activity in tetramer cell staining assays.

Abstract: Immunoassays useful for detecting free &bgr;2-microglobulin in a sample containing &bgr;2-microglobulin/&bgr;2-microglobulin associated protein complexes are provided. Also provided are a sandwich immunoassay and a competition immunoassay for detecting free &bgr;2-microglobulin in a sample containing MHC monomers or MHC tetramers. Kits for performing such immunoassays also are provided.