The major histocompatability complex () includes a variety of genes in vertebrates, which are responsible for the histocompatibility of tissues in transplantations, due to their function in immune recognition, amongst others. -protein complexes are expressed, and act as antigens, on the surface of every cell that is involved in immunological functions. There are two major classes of with distinct pathways and functions.
The pathway allows recognition and elimination of dysfunctional cells by T-cells (killer cells). The can be found on almost all nuclei surfaces of the organism and serve as presenters of antigens in order for T-cells to recognize them for termination.
are presented by antigen presenting cells (APC) and allow T-cells to recognize them. Monocytes and macrophages, amongst others, belong to the APC. They move to the endoplasmatic reticulum (ER) and are associated with the invariant chain. The latter associates with another invariant chain constituting a homotrimer. This complex is bound to and leaves the ER.
Proteases cleave the invariant chains in multiple locations, leaving behind the class-II-associated invariant chain peptide (CLIP) in the binding pocket. If dissociated the becomes active.
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