ANATOMY

Detailed explanation-1: -An antigen-presenting cell (APC) or accessory cell is a cell that displays antigen bound by major histocompatibility complex (MHC) proteins on its surface; this process is known as antigen presentation. T cells may recognize these complexes using their T cell receptors (TCRs).

Detailed explanation-2: -There are two types of MHC: MHC class I and MHC class II. MHC class I presents to cytotoxic T cells; MHC class II presents to helper T cells.

Detailed explanation-3: -CD4 binds to the MHC class II molecule and CD8 to the MHC class I molecule. During antigen recognition, depending on the type of T-cell, CD4 or CD8 molecules associate on the T-cell surface with the T-cell receptor and bind to invariant sites on the MHC portion of the composite MHC:peptide ligand.

Detailed explanation-4: -MHC is the tissue-antigen that allows the immune system (more specifically T cells) to bind to, recognize, and tolerate itself (autorecognition). MHC is also the chaperone for intracellular peptides that are complexed with MHCs and presented to T cell receptors (TCRs) as potential foreign antigens.

Detailed explanation-5: -Although undetectable on resting T cells, MHC class II molecules are expressed to a high level on activated T cells. The role of class II molecules constitutively expressed on antigen-presenting cells (APC) is well documented with regard to antigen presentation to CD4+ T cells.