Final answer:
The HLA-A, HLA-B, and HLA-C proteins are part of the Major Histocompatibility Complex (MHC) I molecules, which are responsible for presenting endogenous antigens to cytotoxic T cells and are found on the surface of nearly all nucleated cells.
Step-by-step explanation:
Major Histocompatibility Complex (MHC) Molecules
The proteins HLA-A, HLA-B, and HLA-C belong to the Major Histocompatibility Complex (MHC) I molecules. The MHC is a collection of genes coding for glycoproteins that are expressed on the surfaces of all nucleated cells. MHC I molecules consist of a longer α chain that is coupled with a smaller β₂ microglobulin protein. These molecules are crucial in the immune system's recognition of self-cells and the presentation of antigens to T cells. The antigen-binding cleft of MHC I is formed by the α1 and α2 domains and is where antigens are loaded for recognition by T lymphocytes.
In comparison, MHC II molecules are primarily found on the surface of antigen-presenting cells. MHC II molecules are composed of two approximately equal length chains, α and β, both of which span the plasma membrane. Unlike MHC I, MHC II's antigen-binding cleft is formed by the α1 and β2 domains.
The HLA-A, HLA-B, and HLA-C proteins are not part of the complement system, MHC II, MHC III, or red blood cells surface proteins. Instead, they play a vital role in immune response by presenting endogenous antigens, which are derived from within the cell, to cytotoxic T cells.