Pathophysiology Major Histocompatibility Complex

The primary target of the immune response against a transplanted organ is the major histocompatibility complex (MHC).1, The MHC is a region of highly polymorphic genes located on the short arm of chromosome six. The human MHC is referred to as human leukocyte antigen (HLA). HLA are a set of glycoproteins that are expressed on the surface of most cells. These proteins are involved in immune recognition, which is the discrimination of self from nonself, but are also the principal antigenic determ-

inants of allograft rejection. '

The protein products of the HLA have been classified into two major groups, Class I and II:

• Class I: these molecules are expressed on the surfaces of all nucleated cells and are recognized by CD8+ cells, also known as cytotoxic T cells.

• The three subclasses of MHC Class I are HLA-A, HLA-B, and HLA-C.

• Class II: these molecules are expressed solely on the surfaces of antigen-presenting cells (APCs). The APCs serve to stimulate CD4+ cells, also known as helper T cells.

• The three subclasses of MHC Class II are HLA-DP, HLA-DQ, and HLA-DR. T and B Lymphocytes

Lymphocytes are one of the five kinds of white blood cells. Mature lymphocytes are astonishingly diverse in their functions. The most abundant of the lymphocytes are T lymphocytes (also called T cells) and B lymphocytes (also called B cells).

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