A variety of molecular components of the adaptive immune system bind epitopes (antigenic determinants). Compare and contrast the binding of epitopes by antibody molecules, T cell receptors (TCRs), and MHC molecules, and describe the consequences of the different interactions

Question

What will be an ideal response?

Answer 1

MHC molecules bind processed antigen for presentation to lymphocytes. The MHC molecules are also composed of two peptides, analogous to the TCR. MHC I molecules bind endogenous antigenic determinants, that is, portions of molecules synthesized by the cell. MHC I-epitope complexes on the surface of cells are detected by T lymphocytes, leading to the activation of Th1 lymphocytes, or cytotoxic attack on the presenting cell by cytotoxic T lymphocytes. MHC II molecules bind antigen fragments produced by digestion of antigens in phagolysosomes (exogenous antigens) and display them to T lymphocytes, which become activated Th2 lymphocytes. Only antigens that MHC molecules can bind will elicit a T-cell mediated immune response.
T cell receptors (TCRs) are produced by T lymphocytes for detecting processed antigen. The TCR is composed of two peptides. When T lymphocytes encounter presented antigen for which they are specific, they become activated and participate in the adaptive immune response. Helper T lymphocytes produce the interleukins needed to regulate the immune response, and cytotoxic T lymphocytes become killers. Thereafter, cytotoxic T lymphocytes use their TCR to identify target cells to attack. Both MHC and TCR are embedded in the cytoplasmic membrane of the cell expressing them.
Antibody molecules produced by B lymphocytes are weapons that either directly damage the antigen they bind or facilitate leukocyte attack on their attached antigen. An antibody molecule epitope binding site is formed by the "variable" regions of two peptides, the heavy chain, and the light chain. Antibody molecules are secreted by the plasma cells into the fluid compartments of the body. The sole exception is IgM, which functions as an antigen receptor on unactivated B lymphocytes.