Antigen presentation; T cell mediated immunity

Protein antigens are no able to induce an immune response without being previously processed by antigen presenting cells (APCs). Following their processing that comprises their splitting to smaller fragments – peptides, APs subsequently present them to T cells; moreover, they activate them and polarise to a specific biological functions. Depending of antigen origin, there are two presentation pathways, exogenous and endogenous. Antigens originated from outside of APC, e.g. bacterial toxins, enzymes, etc., are presented by exogenous pathway and presented molecules are class II HLA molecules. T cell, that recognise presented peptides belong to helper subset of T cells. Antigens originated in the cytosol, such as antigens that appear in the cytoplasm of virus infected cells, are presented by endogenous pathway and presented molecules belong to class I HL-A molecules. T cells, that recognise presented peptides, represent cytotoxic T cells.

 

T cells are not unique population of cells. There are two basic subpopulations, those that express αβ T cell receptors and those that express γσT cell receptors. αβ T cells also are not unique, we can distinguish there basic subsets: helper, cytotoxic and regulatory. Moreover, T helper cells comprise of further subsets: TH1, TH2, and TH17, respectively. Each subset mediates different biological effector functions, such as activation of macrophages, NK cells, provide a help to B and T cytotoxic cells, etc. Effector activity of T cells, as well as APC, B cell and other cell of the immune system, is regulated by regulatory T cells (Tregs). There are of two different subsets – natural (nTregs) and induced (iTregs). nTreg cells are independent population of cell “born” in the thymus. They inhibit activity of autoreactive T cells. On the contrary, induced Treg cells appear during the ongoing immune response only and keep it within physiological borders. During the immune response memory T cells (Tm) appear, too. They remember their inducers and subsequent introduction of the antigens induces very prompt and extensive immune response. There are two subsets of Tm cells, either central or effector memory T cells. γσT cells differ from their counterparts, they respond to various exogenous and endogenous antigens that for their activation do not require the processing in APCs.

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