Figure 1

The MHC regions of (a) humans (HLA) and (b) mice (H-2). Only some of the key immune-system genes are shown, out of the more than 200 loci within the MHC. Historically, the MHC has been divided into three regions - class I, class II and class III. Most of the important phenotypes, such as transplant rejection, are associated with the class I and class II loci; these genes are highly polymorphic and some of them have over 300 alleles. Bf, C2 and C4 are complement proteins; LMP, low-molecular weight polypeptide; MIC, MHC class I chain-related; TAP, transporter associated with antigen-processing; TAPBP, TAP-binding protein (tapasin); TNF, tumor necrosis factor.