Chapter 2 – MHC Class II

This chapter provides an overview of major histocompatibility complex Class-II ((MHCII) molecules. The MHCII molecules serve to bind antigenic peptides and engage CD4 T cell receptors to initiate an immune response. MHCII is both polygenic and polymorphic. In mice, two genes, I-E and I-A, code for MHCII, and in humans, three genes, HLA-DP, -DO, and -DR, are present. For each copy of each gene any of a large number of alleles can be expressed, which results in the polymorphic nature of MHCII. Alleles differ from one another by up to 20 amino acids and, as such, are the most highly polymorphic genes known. Polymorphisms, most of which are on the exposed surfaces including the peptide-binding groove, account for critical diversity within the population and are the cause for restriction. The chapter discusses regulation of MHCII expression. It introduces the basic idea of alternative antigen presentation pathways. A brief description of intracellular events and T cell selection is given.