The immune system is the system of specialized cells and organs that protect an organism from outside biological influences. (Though in a broad sense, almost every organ has a protective function - for example, the tight seal of the skin or the acidic environment of the stomach.) 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.