12. 6 Major Genes: 10,000 Antigens
Six important genes for human leukocyte antigens are contributed by each parent. So each person inherits a double set of six major genes that produce six major corresponding antigens, HLA-A, -B, -C, -DP, -DQ, and -DR. Since as many as 20 varieties exist for each of these HLA-producing genes in different individuals, the total possible human leukocyte antigens that can mark "self" and trigger an immune response number about 10,000!