Antigen—MHC complex

T-cell activation involves interactions of membrane-bound immunoglobulin-like receptors with membranous antigen-MHC complexes. The antigen receptors of B and T lymphocytes are complex membrane-bound multisubunit proteins. Antigen receptors are immunoglobulin (Ig)-like antibody molecules. B cells do not secrete the first antibodies that they make, instead, they insert them into the plasma membrane, where they now serve as receptors for antigen. Each B cell has approximately 10 such molecules in the plasma membrane. 

Madden, D.R., Garboczi, D.N., Wiley, D.C. The antigenic identity of peptide-MHC complexes a comparison of the conformation of five viral peptides persented by HLA-A2. Cell 75 693-708, 1993. 

Quiescent T-lymphocytes are stimulated largely by direct binding to an antigen fragment presented on the surface of a macrophage in the context of MHC complex (Figure 9.3). This results in the induction of expression of at least 70 genes whose products are collectively important in immune stimulation. These products include ... 

When a small antigen is complexed with the MHC protein it is protected from proteolysis or other damage (e.g. effects of free radicals) within the presenting cell. 

Fig. 11.1. Principle of an immunological synapse. Possibilities for communication between B and T cells during an immune response. Antigenic peptides are presented by the MHC complex class II at the surface of the B cell. The antigens are recognized and bound by T cell receptors of the T cell. The T cell receptor is activated and sets a signal chain in motion that leads to activation of the expression of cytokines, such as IL-2. The cytokine is secreted, and binds and activates a cytokine receptor on the B cell. TNFa is shown as another example of a ligand-receptor system. TNFa communicates, as a membrane-bound ligand, with a corresponding receptor on the surface of the B cell. The interactions shown take place in a narrow spatial region between B and T cells, which is why this system is referred to as an immunological synapse. TNF tumor necrosis factor MHC major histocompatibility complex IL-2 interleukin 2.

Antigen MHC class I complexes have a different function and present their antigen to CD8+ cytolytic T cells, which produce an appropriate group of effector molecules leading to apoptosis of target cells. 

The MHC complex and antigen can then be detected such as by CD8 cytotoxic T cells and the cells destroyed. However, the fragments of the hapten complex can also be detected by other cells such as APC, and with the collaboration of T-helper cells, B cells would become activated and antibodies produced. These could then bind to the surface antigen as described for a type II reaction. 

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