A T Borchers, C L Keen and M E Gershwin,
University of California at Davis, Davis, CA, USA
© 2005 Elsevier Ltd. All rights reserved.
Immunity can be defined as the ability of an organism to resist or eliminate potentially harmful foreign organisms and materials or abnormal cells. Any substance capable of eliciting an immune response is called an antigen (antibody generator). Immune responses can be classified as innate or adaptive. Innate immune responses are also called nonspecific because they can be elicited by a wide range of foreign substances and are the same regardless of the exact nature of the substance and whether it had been encountered before. The major mechanisms of innate immunity include phagocytosis, inflammation, complement activation, and induction of cell death. Neutrophils and macrophages are the main cell types responsible for phagocytosis, and the chemical messengers that they and some other cell types produce play an important role in the initiation of an inflammatory response. The induction of apoptosis (programmed cell death) as part of the innate immune response is accomplished by natural killer (NK) cells.
In contrast to innate immune responses, adaptive immune responses are highly specific for a particular antigen and become stronger and more rapid over time. B cells and T cells represent the two types of lymphocytes responsible for adaptive immune responses. The main function of B cells is to produce antibodies, which neutralize pathogens or stimulate their elimination by other cell types through opsoni-zation or complement activation. There are two major classes of T cells, namely helper T cells and cytotoxic T cells. One subclass of helper T cells provides help to macrophages in killing pathogenic microorganisms they have engulfed. The other subclass of helper T cells is vital for the induction of antibody production by B cells. Cytotoxic T cells directly eliminate infected cells by inducing them to undergo apoptosis. T cells also play a central role in self-tolerance (i.e., the ability not to respond to self antigens).
Initial exposure to pathogens (i.e., disease-producing microorganisms such as viruses and bacteria) most commonly occurs at the interfaces of host tissues and the external environment. Such tissues include the outer cells of the skin and, since the vertebrate body is essentially a 'tube within a tube,' the layers of cells and mucous lining the digestive, reproductive, and respiratory tracts. These cell layers and their secretions constitute nonimmunological physical and chemical barriers that provide a first line of defense against invasion by pathogenic microorganisms. Their barrier function is often reinforced by a variety of bacteria that generally do not harm the host but, on the contrary, provide additional protection from pathogens via competition, production of toxic substances, and stimulation of the immune system.
The main function of the immune system is to provide protection from invading pathogens, primarily viruses and bacteria but also fungi and parasites. For this purpose, the ability to discriminate between self or harmless non-self and potentially harmful non-self is absolutely crucial. Also important is the capability to recognize whether pathogens are extracellular (outside of the host's cells), such as fungi, certain bacteria, and some parasites, or intracellular, such as other bacteria and parasites and all viruses. Other activities of the immune system include the removal of worn-out cells, the identification and destruction of mutant or otherwise abnormal cells and also such inappropriate responses as allergies and autoimmune diseases, and graft rejection after organ transplantation.
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