11.1 Antigen recognition by T lymphocyte

In order to generate an acquired immune response an antigen molecule must be broken inside the cells and presented to the immune cells with the help of major histocompatibility complex (MHC) molecules. These are encoded by the genes of MHC complex and vary between different species. Antigens can trigger an immune response only after bounding to MHC molecules. All vertebrate animals contain the MHC encoding loci in their chromosomes.

Most of the T lymphocytes can recognize the small peptide fragments while the B cells can recognize the peptides, carbohydrates, lipid, nucleic acid and other chemicals. Because of different antigen specificity of T and B lymphocytes, cell mediated immune responses are usually activated by a protein antigen while humoral immune responses are activated by non-protein antigens.

The T cells recognize only protein antigens displayed by MHC molecules because the MHC cannot bind to any other molecules. An individual T cell can recognize only one specific MHC molecules loaded with the peptide, the property is called as MHC restricted .

T cells can recognize only the linear peptides and not the conformational epitopes of an antigen because the conformations of the proteins are lost during the processing and loading into the peptide binding cleft of MHC molecules.

T cells can recognize only the antigens that are associated with the antigen presenting cells and not to the soluble protein.

11.2 Antigen presenting cells

Many cell types function as antigen presenting cells to activate the naïve and effector T cells. Dendritic cells are the most common and effective antigen presenting cells in the body. Macrophages and B cells also act as an antigen presenting cells, but only to the previously activated T cells. All the above mentioned cells expresses the MHC type II molecule over their surface and hence also called professional antigen presenting cells . The antigen presenting cells displays the peptide MHC complex to T cells and also provides the additional stimuli to T cells for its proper functioning. These stimuli are sometimes called as costimulatory molecules because they function together with the antigen presenting cells.

The antigen presenting function of the antigen presenting cells can be enhanced by microbial products. The induction of T cell response against an antigen is usually enhanced by the administration of purified protein products called as adjuvants . Adjuvants are derived from microbes such as killed mycobacterium which mimics the microbes and stimulate the production of immune response.