A defence system found in organisms to protect against danger, including sterile injury, foreign organisms, such as viruses and bacteria, and cancer. The effector cells that form the immune system include the leucocytes, such as neutrophils and macrophages, which engulf and digest cell debris and foreign matter by phagocytosis, and lymphocytes, which aid the recognition of invading organisms. An immune response is triggered when the immune system recognizes danger signals (microorganisms or alarm signals released by damaged cells).
The immune system can be subdivided into the innate and adaptive systems. The innate system, the first line of defence, includes anatomical features that function as barriers to infection and provides a non-specific set of responses. In contrast, the adaptive system provides a highly specific response to a pathogen because of the ability of certain lymphocytes (in particular the B cells that mature into plasma cells) to produce antibodies that selectively bind to pathogen-specific antigens.
Both systems have cellular and humoral components; the latter referring to the production of cytokines and antibodies. Immune deficiency viruses, including HIV, attack the immune system and render the animal susceptible to secondary infections to produce the symptoms of AIDS. Problems frequently arise during transplant operations in which tissue from a donor transplanted into the recipient patient is rejected as a result of the host immune system's ability to recognize foreign cells.
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