ANTIGEN PRESENTING CELLS (APCs)

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Antigen processing is the immunological process involved in the digestion or breakdown of antigenic molecules to release peptide molecules that are associated with MHC molecules and presented on the surfaces of the infected host cells for presentation to effector lymphocytes particularly the T cells. Antigen processing is the series of immunological events that is mainly involved in the formation of antigenic peptide molecules complexed with MHC molecules (Class I and Class II MHC molecules inclusive) for rapid immunological response. The ability of the immune system to process an antigen and make it available for further specific immunological response is vital to the elimination of the foreign body or pathogen that invaded the host’s body. Antigen processing is important for the elicitation of an appropriate and specific immune response. Depending on their portal of entry into the body, the antigen processing arm of the immune system ensures that immunogens or antigens are properly presented to the effector molecules of the immune system for an enhanced immune response against them. Antigen processing help to unfold intracellularly hidden parasites on the surfaces of infected host cells.

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Macrophages which also act as antigen presenting cells are typical examples of antigen processing cells of the immune system that parades the blood circulation or body, looking for antigens or pathogens which they ingest and digest to produce antigenic peptide molecules that are recognized by effector T lymphocytes. Generally, antigen processing is required to produce antigenic peptide molecules that interact with specific MHC molecules. While antigen processing is mainly involved in the formation of antigenic peptide molecules complexed with MHC molecules, antigen presentation is the sequence of immunological reactions involved in the display of antigenic peptide molecules complexed with MHC molecules on the surfaces of antigen processing cells (APCs).

After their entry into the body, antigens or pathogens are ingested and degraded by antigen processing cells of the immune system including the macrophages; and peptide fragments from the degraded antigen is displayed on the cell surface of the macrophages in association with MHC molecules either Class I MHC molecules or Class II MHC molecules. It is noteworthy that the Class I MHC molecules present antigens to T-cytotoxic cells (CD8+) while Class II MHC molecules present antigens to T-helper cells (CD4+). The main biological function of the Class I and Class II MHC molecules is to bind antigenic peptide molecules or fragments from degraded antigens and bring them to surfaces where they can be recognized by the appropriate T lymphocytes especially the CD4+ and CD8+ cells.

TYPES OF ANTIGEN PRESENTING CELLS (APCs)

APCs are specialized cells of the immune system whose main function is to ingest, process, and present processed antigens in association with any of the MHC molecules to specific arms of the immune system especially the T lymphocytes for further immunological response. Some pathogenic microorganisms including but not limited to viruses, protozoa and some obligate intracellular bacteria (e.g. Chlamydia) expresses their virulence or pathogenicity within their infected host cells where some components of the immune system (e.g., immunoglobulins) cannot easily reach, for immunological response. For such intracellularly-placed pathogens (e.g. viruses) to be attacked by the immune system, the MHC molecules which is normally located on the cell surfaces of normal host cells inclusive of infected host cells displays peptide fragments resulting from the degradation of the intracellular parasite; and such peptide-MHC complexes are the antigens to be recognized by effector molecules of the T lymphocytes especially the CD8+ cells whose main function is to kill viral infected cells and other intracellular parasites. Such antigens are generally referred to endogenous antigens because they resulted from cells infected by pathogens that live inside host cells (e.g., viruses and protozoa).

Those antigens that do not invade host cells (e.g., most pathogenic bacteria) but end up in the lymph nodes where they could easily be attacked or ingested by phagocytes and other components of the immune system such as antibodies are generally called exogenous antigens because they do not reside within host cells. The B cells can recognize local antigens (i.e., pathogens that do not reside within the host cells) without the assistance of the MHC molecules but the T lymphocytes cannot recognize antigens on their own unless such antigens have been processed by APCs and presented in conjunction with any of the MHC molecules either Class I or Class II MHC molecules. Therefore, the processing and presentation of antigens by the APCs is critical for the recognition of the immunogen by the T cells.

Macrophages, dendritic cells and B cells are typical examples of antigen presenting cells (APCs). These APCs recognize, bind or capture and display processed antigenic molecules on their cell surfaces – where other specific components of the immune system such as the T lymphocytes can effectively neutralize them. They are generally known as professional APCs because they display peptide fragments (from antigens) complexed with Class II MHC molecules to T helper (TH) cells. This is important in immune response to a particular antigen because TH cells help B cells to proliferate and secrete numerous antibodies; and this further elaborate the immunological synergy that exist between the humoural or antibody mediated immunity (AMI) – whose main effector molecule is the B cell and the cell-mediated immunity (CMI) – whose main effector is the T lymphocytes. Non-professional APCs which include thymic or thyroid epithelial cells, skin fibroblasts and brain grail or microglia cells amongst others are APCs that are usually stimulated to express Class II MHC molecules on their cell surface membranes. Non-professional APCs only last for a short period of time in an immune response especially in inflammatory reactions in the body. 

Further reading

William E.P (2003). Fundamental Immunology. 5th edition. Lippincott Williams and Wilkins Publishers, USA.

Stevens, Christine Dorresteyn (2010). Clinical immunology and serology. Third edition. F.A. Davis Company, Philadelphia.

Silverstein A.M (1999). The history of immunology. In Paul, WE (ed): Fundamental Immunology, 4th edition. Lippincott Williams and Wilkins, Philadelphia, USA.

Paul W.E (2014). Fundamental Immunology. Seventh edition. Lippincott Williams and Wilkins, USA.

Male D, Brostoff J, Roth D.B and Roitt I (2014). Immunology. Eight edition. Elsevier Saunders, USA.

Levinson W (2010). Review of Medical Microbiology and Immunology. Twelfth edition. The McGraw-Hill Companies, USA.

Berzofsky J.A and Berkower J.J (1999). Immunogenicity and antigen structure. In Fundamental Immunology, 4th edition., W.E. Paul, ed., Lippincott-Raven, Philadelphia. 

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