Cell-mediated immunity is an immune response that does not involve antibodies or complement, but rather involves the activation of macrophages, natural killer cells, antigen-specific cytotoxic T-lymphocytes, and the release of various cytokines in response to an antigen.
Historically, the immune system was separated into two branches, cellular immunity; for which the protective function of immunisation was associated with cells. CD4 cells or helper. T cells provide protection against different pathogens and humoral immunity, for which the protective function of immunisation could be found in the humour (cell-free body fluid or serum).
Mechanism
Cellular immune response is also called T-cell immunity as it occurs by the T-cells or T-lymphocytes (figure). This immune response defends against pathogens invading the host cells. The surface of T-cells has receptor molecule that binds with antigens. A single T-cell has about 100,000 receptor sites.
An antigen on entering the body is first attacked by the macrophages and gets fragmented into pieces. It then presents a piece of antigen to the T-helper cells, which recognise the antigen and trigger a series of cellular response. A clone of T-lymphocytes is first formed after getting activated by the T-helper cells. There are different kinds of T-cells, which are morphologically similar but functionally different.
The principal cells involved in cellular immunity and their functions are given in table :
| Cell | Functions |
| Helper T cells (TH) | Necessary for B-cells activation by T-dependent antigens. |
| Suppressor T cells (TS) | Regulates immune response and helps in maintaining immune tolerance. |
| Delayed hypersensitivity T cells (TD) | Provides protection against infectious agents; causes inflammation in association to tissue transplant rejection. |
| Cytotoxic T cells (TC) | Destroys target cells on contact. |
| Killer cells (K) | Attacks antibody-coated target cells. |
| Natural Killer cells (NK) | Attacks and destroys target cells. |
Importance :
Cellular immunity protects the body by:
Activating antigen-specific cytotoxic T-lymphocytes that can induce apoptosis in body cells displaying epitopes of foreign antigen on their surface, such as virus-infected cells, cells with intracellular bacteria, and cancer cells displaying tumour antigens;
Activating macrophages and natural killer cells, enabling them to destroy intracellular pathogens; and
Stimulating cells to secrete a variety of cytokines that influence the function of other cells involved in adaptive and innate immune responses.
Cellular immunity is directed primarily at microbes that survive in phagocytes and microbes that infect non-phagocytic cells. It is most effective in removing virus infected cells, but also participates in defending against fungi, protozoans, cancers, and intracellular bacteria. It also plays a major role in transplant rejection.
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