General features of the immune system

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Immunity refers to the body's resistance to invasive pathogens (viruses, bacteria, fungi, protozoa and multicellular parasites) or their toxic products, to allergens

(pollen, animal hair, chemicals, etc.) and to unwanted cells or cell products arising within the body from cancer or autoimmune diseases. The immune system comprises (1) cellular defence mechanisms mediated by several types of leucocytes and (2) humoral defence mechanisms mediated by soluble proteins, so-called because they are dissolved in the body fluids rather than being primarily associated with cells. Any immune response involves, firstly, recognition of the pathogen or other foreign material and, secondly, elimination of these invaders. When an immune response occurs in an exaggerated or inappropriate form, the term 'hypersensitivity' is applied. The fundamental concept underlying the function of the immune system is the ability to distinguish, at the molecular level, 'self' from 'non-self' (foreign) materials.

All cells of the immune system originate from just one cell type in the bone marrow of adult mammals, the haemopoietic stem cell. These pluripotent cells give rise to two main lineages: (1) the lymphoid lineage produces lymphocytes and (2) the myeloid lineage produces mononuclear and polymorphonuclear phagocytes, megakaryocytes (precursors of platelets) and mast cells. The precise origin of natural killer cells and dendritic cells is uncertain, although they do develop ultimately from haemopoietic stem cells.

With regard to pathogens, the immune response depends on the site of infection and the nature of the pathogen. All viruses, some bacteria and some protozoan parasites replicate inside host cells, whereas many bacteria and larger parasites replicate in extracellular spaces and body fluids. To clear an intracellular infection, it is necessary to destroy the infected host cells. Extracellular pathogens are selectively destroyed and their toxic products neutralized.

Immune responses may be either innate (natural) or acquired (adaptive). Innate immunity, being genetically determined, is present from birth and is in full readiness for an attack by invading pathogens. The innate defence mechanisms act non-specifically against a wide range of microorganisms and foreign material and can be mobilized at the site of infection within hours. The main characteristics and components of innate immunity occur in the inflammatory response. There is no immunological memory; that is, the response is not dependent upon prior exposure to a particular infectious agent.

Acquired immunity is not immediately ready to combat a foreign invader that has never previously entered the body; it develops over a long period only after invasion by a novel intruder. However, once an acquired immune response has occurred, the body acquires the capacity to recognize and destroy that particular invader very rapidly the next time it is encountered, and the individual is now said to be immune to it. Not only is there an immunological memory, but the response to the particular invader on subsequent occasions is much more vigorous and effective than the response to the first encounter.

It is well documented that a decline in immune function takes place with advancing age (Walford, 1980).

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