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.

Types of immune system

Immune systems for the animate kingdom may conveniently be divided into two types:

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• Bacteria and monocellular organisms have an 'immune system' that is designed to combat bacteriophages (viruses that infect bacteria). They do this by simultaneously producing restriction enzymes that destroy DNA at certain sequences, and enzymes that protect their own DNA from this enzyme (by methylating the same sequence). The bacterium's DNA will not, therefore, be damaged by the restriction enzyme because of the presence of the second enzyme. However, when a bacteriophage attempts to infect this bacterium, its viral DNA has not been protected, and gets degraded by the restriction enzyme. While studies of the bacterial immune system provide useful insights into immunology, higher organisms (such as mammals) have far more complex immune systems.
• In multicellular organisms, the immune system is an organ system that acts, not only as a defense against foreign pathogens (such as viruses, bacteria, parasites), and some poisons, but also in suppressing cancer cells.