Immune system:

The immune system is responsible for defending the organism from various pathogens; it also is responsible for the elimination of cancer cells. This through a series of processes such as the activation of white blood cells, inflammatory processes, among others.

A healthy immune system is able to respond correctly to pathogens. On the other hand, a depressed one is unable to contain infections, as is the case of people with  immunodeficiency. The opposite, it can overreact, as is the case of  allergies. Or, being unable to distinguish between antigens and the body's own cells; such is the case of  autoimmune diseases.