The immune system is a collection of mechanisms within an organism that protects against disease by identifying and killing pathogens and tumor cells. It detects a wide variety of agents, from viruses to parasites, and needs to distinguish them from the organism's own healthy cells and tissues in order to function properly.

In humans, the immune system consists of many types of proteins, cells, organs, and tissues, which interact in an elaborate and dynamic network. The immune system adapts over time to recognize particular pathogens more efficiently. This adaptation creates immunological memories and allows even more effective protection during future encounters with these pathogens.

Immunodeficiency diseases occur when the immune system is less active than normal, resulting in recurring and life-threatening infections. Immunodeficiency can either be the result of a genetic disease, such as severe combined immunodeficiency, or be produced by pharmaceuticals or an infection, such as the acquired immune deficiency syndrome (AIDS) that is caused by the retrovirus HIV. In contrast, autoimmune diseases result from a hyperactive immune system attacking normal tissues as if they were foreign organisms. Common autoimmune diseases include rheumatoid arthritis, diabetes mellitus type 1 and lupus erythematosus.

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