Exploring the Human Immune System

2.4.1 Overview

The immune system can be subdivided into two systems; innate immunity and adaptive immunity. Innate immune system is made up of physical, chemical, and microbiological barriers, as well as the others elements of the immune system such as phagocytic cells (polymorphonuclear neutrophils, monocytes, macrophages), cells that release inflammatory mediator (basophils, mast cell, and eosinophils), natural killer cells, and the protein component (complement, cytokines, and acute phase proteins). Innate immune responses provide immediate host protection towards infectious agents that invade host without an immune memory and specificity (Delves and Roitt 2000b; Delves and Roitt 2000a; Parkin and Cohen 2001; Roitt et al. 2001).

Adaptive immune responses involve both antigenic specific T cells and B cells. Lymphocytes responses constitute of sequenced phases commencing from cellular activation, proliferation and effectors functions. Cellular proliferation is a vital phase whereby activated lymphocytes are amplified into sufficient copies of antigen specific cells in order to perform effector function as cytotoxic cells. The priming events usually take place in the specialized environment of lymphoid tissue and aid by a specialized antigen presenting cells. Activated lymphocytes will then carried out their effectors functions; including cytokine production, cytotoxicity, and antibody synthesis. Adaptive immune responses can sub-divide into cell-mediated immunity (helper T cells secrete cytokine or/and soluble factors to mediate the immune responses; cytotoxic T cells release lymphotoxin which lyses target cell) and humoral immunity (B cells will differentiate into plasma and secrete antibodies). (Delves and Roitt 2000a; ...

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...n important step in selecting effectors functions according to their profile of cytokine production (Figure 3). Th1 cells secrete IFN-γ, and IL-2 which will promote the macrophage activation, CTL formation, and antibody-dependent cell mediated cytotoxicity. Th2 cells, on the other hand, produce IL-4, IL-5, IL-6, and monitor the IgG1 and IgE isotype switching and mucosal immunity, stimulation of mast cells and eosinophils growth, and IgA synthesis. Thus, Th1 cells are contributing the basis of cell-mediated inflammatory reaction while Th2 cells help in humoral immune responses- the antibody-antigen and allergic reactions. Actions of Th cells populations are interconnected, activation of one subset will inhibit anothers. T cells can also be primed by some mitogenic substances such as PHA (phytohaemagglutinin) or concanavalin–A, regardless of their antigen specificity.