CELL-MEDIATED IMMUNITY (CMI)

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Cell-mediated immunity (CMI) is the type of specific or adaptive immunity that is mediated by T lymphocytes. They are mainly responsible for protecting the body against infectious disease agents that invade the host cells such as viruses and parasites or protozoa including cancer or tumour cells. Various types or subpopulations of T cells (known as effector T cells) exist. These T cell subpopulations mediate CMI in animal or human hosts. Of particular importance are the cytotoxic T cells (CD8+ cells) which kill virus-infected cells, tumour/cancer cells, or transplanted tissue cells; and helper T cells (CD4+ cells) which participate in antigen recognition and other immune system functions or regulation such as assisting the B cells in antibody secretion.

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Both T cells and B cells interact together and with other antigen presenting cells in a bid to contain the activities of antigens or pathogens that enter the body. Though antibodies produced during humoural immune response has a role to play in containing the spread of viral infection in the body, the components of the CMI especially the cytotoxic T cells are the major immunological factors that defend against viral infections in the body. A dysfunctional cell-mediated immunity (CMI) in a human host predisposes the individual to several viral and protozoal infections and other intracellular bacterial infections since the components of CMI are mainly responsible for defending the host against similar antigens.

T lymphocytes unlike the B cells do not recognize antigens on their own without the help of major histocompatibility complex (MHC) molecules. MHC molecules as earlier highlighted, mediate the detection of peptide fragments from antigens in host cells (inclusive of viral infections, fungal infections and protozoal infected cells). MHC molecules (including class I or class II MHC molecules) bind to peptide fragments of antigens digested within the infected host cell and present these antigenic peptide molecules on the surface of the target cell (in this case the infected host cell harbouring the pathogen or antigen).

The display of these peptide fragments of antigens on the surface of the infected cells by the MHC molecules facilitates the recognition of these antigenic peptide molecules by the T cells, and this supports the destruction of the antigen by the coordinated mechanisms of the immune system. CD8+ cells generally respond to antigens in association with MHC I molecules (class I MHC molecules) on the surface of a target or host cell while the CD4+ cells respond to antigens in association with MHC II molecules (class II MHC molecules) on the surface of host cells. There is plethora of T cell subpopulations but a summary of some of the key subpopulations of T cells and their biological functions are shown in Table 1.

Table 1. Summary of the biological function of T cell subpopulation

T cell subpopulationImmunological function
  
Cytotoxic T lymphocytes (CTLs)CTLs which can also be called *CD8+ or T cytotoxic (Tc) cells are effector T cells that mediate the lysis or killing of infected target cells that bear antigenic peptide molecules in association with Class I MHC molecules including cancer cells.   
T helper (TH) cellsTH cells also called CD4+ T cells are effector T cells which do not take part in the direct killing of virus or parasite-infected host cells but they participate in the recognition of antigenic molecules especially those complexed with the Class II MHC molecules. TH cells have subsets of TH1 and TH2 cells; and they both take part in the production of cytokines including interleukins (IL) which stimulate the immunological response of the T cells. TH cells help B cells to proliferate and secrete numerous antibodies.  
T suppressor (TS) cells  TS cells are subpopulations of T cells that mainly regulate the cell-mediated immune response by suppressing other effector cells of the immune system.  
TDTH cells  TDTH cells are delayed-type hypersensitivity T cells whose main function is to release cytokines and chemokines after interacting with antigens especially against intracellular parasites and bacteria that stimulate allergic reactions in human hosts. TDTH cells are mainly produced in Type IV hypersensitivity reactions.    
CD3 cellsCD3 cells are mainly signaling transduction molecules that are found on the surfaces of host membranes especially at the T cell receptors (TCRs). 
  CD 28  CD 28 cells are T cell membrane molecules that play significant role in helping the immune system to discriminate between self-molecules and non-self molecules.
  

*CD= Cluster of Differentiation. CD generally represents cell surface molecules or markers used in defining leukocytes especially the T cells based on an internationally recognized system of classification. Over 300 CD molecules are known that coat the surfaces of both the T and B lymphocytes. CD molecules are subset of cellular surface receptors for epitopes that identify cell type and stage of differentiation of lymphocytes which are recognized by antibodies. CD8+ cells and CD4+ T cells are the most commonly known cluster of differentiation molecules.

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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