[Antigen presentation and the triggering of the immune response]

Immune defense is based on the interaction of nonspecific factors of natural resistance and factors of antigen-specific adaptive immune response. The key event of this interaction is antigen presentation. Its matter is a recognition of antigenic peptide complexed with MHC molecule of class II on the antigen-presenting cell (APC) surface by TCR receptor of T helper cell. The realization of the antigen presentation is connected with some problems: the number of cells in specific T cell clones is too low; the complexes of each peptide with MHC-II is a low part of the general population of APC surface peptide-MHC-II complexes; the bonds forming between these complexes and TCR molecules are too weak and some other molecules must be involved to reach T cell activation. These difficulties and mechanisms of their overcoming are considered in the review.