ABSTRACT
The immune system has evolved under the influence of continuous challenge by pathogens, most of which have numerous ways of evading the host defence. The first “trench” of defence is provided by cells and molecules of the “innate” or “natural” immunity, including phagocytes, T cell receptor (TCR) yδ + T cells, natural killer (NK) cells, mast cells and eosinophils, as well as complement components and proinflammatory cytokines, such as interferons (IFNs), interleukin (IL)-1, IL-6, IL-12 and tumor necrosis factor (TNF)-α. The more specialized TCR αβ + T lymphocytes provide the second defence wall. These cells account for the “specific or adaptive immunity’, which results in specialized types of immune responses which allow vertebrates to recognize and eliminate, or at least control, infectious agents in different body compartments. For example, viruses growing within the infected cell, are faced through the killing of their host cells by CD8+ class I MHC-restricted cytotoxic T lymphocytes. Most of microbial components are endocytosed by antigen-presenting cells (APC), processed and presented preferentially to CD4+ class II MHC-restricted T helper (Th) cells, CD4+ T cells co-operate with B cells for the production of antibodies which opsonize extracellular microbes and/or neutralize their exotoxins. This branch of the specific Th cell-mediated immune response is known as “humoral immunity’. Other microbes, however, survive within macrophages despite the unfavorable microenvironment and antigenactivated CD4+ Th cells are required to activate macrophages, whose reactive metabolites and TNF-α then lead to the destruction of pathogens. This latter branch of the specific Th cell-mediated response is known as “cellmediated immunity” (CMI).
