Immune system

The immune system is the system of specialised cells and organs that protect an organism from outside biological influences. In a broad sense, almost every organ has a protective function (e.g., the skin). When the immune system is functioning properly, it protects the body against bacteria and viral infections, destroying cancer cells and foreign substances. If the immune system weakens, its ability to defend the body also weakens, allowing pathogens, including viruses that cause common colds and flu, to grow and flourish in the body. The immune system also performs surveillance of tumor cells, and immune suppression has been reported to increase the risk of certain types of cancer.

Self and non-self

The immune system defends the body by:

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• recognizing agents that represent 'self' and those that represent 'non-self'; and
• launching attacks against harmful members of the latter group.

Distinguishing between self and non-self, and between harmful non-self and harmless non-self, is a difficult problem, and a variety of mammal disorders (immunodeficiency and autoimmunity) arise from the failures of their discriminatory systems.

Related Topics:
Immunodeficiency - Autoimmunity

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Some self/non-self discrimination is effected by hard-wired mechanisms that recognize features displayed 'only' by pathogens. The mannan-binding lectin pathway of the complement system, for instance, recognizes mannose sugars that appear only in the polysaccharide coats of various species of bacteria.

Related Topics:
Mannan-binding lectin pathway - Complement system - Mannose - Polysaccharide

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The most versatile mechanisms of discrimination, however, are not hard-wired; rather, they involve the immune system 'learning' to recognize non-self. For instance, the plasma membrane of every 'nucleated' cell contains molecules of a large glycoprotein, called the major histocompatibility complex (MHC). These proteins have configurations and amino acid sequences that are unique to every individual. Cytotoxic T cells (T cells that directly destroy cells) contain surface-mounted receptors that are used to determine if a given cell is virally-infected by reading the peptides displayed on its MHC molecules. During their development, T cells are selected for self-reactivity. If a given cell contains receptors that bind strongly to an existing molecule in the body, it is destroyed by forced apoptosis, leaving behind T cells that can be safely released into the body.

Related Topics:
Plasma membrane - Glycoprotein - Major histocompatibility complex - Amino acid - Cytotoxic T cell - T cell - Receptors - Peptide - Apoptosis

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