A transmembrane protein complex composed of an MHC alpha chain and, in most cases, either an MHC class II beta chain or an invariant beta2-microglobin chain, and with or without a bound peptide, lipid, or polysaccharide antigen. [ http://openisbn.com/search.php?q=0781735149&isbn=1/ GOC:jl http://www.ncbi.nlm.nih.gov/pubmed/16153240 http://www.ncbi.nlm.nih.gov/pubmed/15928678 GOC:add ]