Everything about Mhc Class Ii totally explained
MHC (major histocompatibility complex) Class II molecules are found only on a few specialized cell types, including
macrophages,
dendritic cells and
B cells, all of which are professional
antigen-presenting cells (APCs).
The peptides presented by class II molecules are derived from extracellular proteins (not cytosolic as in
class I); hence, the MHC class II-dependent pathway of antigen presentation is called the
endocytic or
exogenous pathway.
Loading of class II molecules must still occur inside the cell; extracellular proteins are
endocytosed, digested in
lysosomes, and bound by the class II MHC molecule prior to the molecule's migration to the plasma membrane.
Structure
Like MHC class I molecules, class II molecules are also
heterodimers, but in this case consist of two homologous peptides, an α and β chain, both of which are encoded in the MHC.
Because the antigen-binding groove of MHC class II molecules is open at both ends while the corresponding groove on class I molecules is closed at each end, the antigens presented by MHC class II molecules are longer, generally between 15 and 24
amino acid residues long.
Reaction to bacteria
Because class II MHC is loaded with extracellular proteins, it's mainly concerned with presentation of extracellular pathogens (for example, bacteria that might be infecting a wound or the blood). Class II molecules interact exclusively with
CD4+ ("helper") T cells (T
Hs). The
helper T cells then help to trigger an appropriate immune response which may include localized
inflammation and swelling due to recruitment of phagocytes or may lead to a full-force antibody immune response due to activation of B cells.
Synthesis
During synthesis, MHC class II is the result of dimerization of α and β chains, with the assistance of an
invariant chain.
Genes
Further Information
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