Figure 1. The histocompatibility antigens encoded by the class-I and class-II loci of the HLA complex. The class-I molecules are expressed on the surface of all nucleated cells in the body; class-II molecules are expressed on a subset of cells, primarily those, such as dendritic cells, macrophages and B lymphocytes that are engaged in antigen processing and presentation. The class-I and class-II loci are highly polymorphic.

Figure 2. Segregation of HLA genes within a family. Because the HLA genes are so closely linked, those present on a given member of a chromosome pair are usually transmitted to the next generation as a unit (called a haplotype). The genes are inherited in a Mendelian fashion so that a given offspring has approximately a one-fourth chance of having a full sibling inheriting the same combination.