The immune system is the means by which the body defends itself from infection and disease. Deficiencies of the immune system (immunodeficiency disorders) mean that the immune system is less active or effective than normal and thus leaves the body at risk for infection from common pathogens (so-called opportunistic infections) that a healthy immune system could fight off. For instance, in the early years of the acquired immune deficiency syndrome (AIDS) epidemic, pneumocystis pneumonia (PCP) was a common cause of death for people with AIDS, while people whose immune system is functioning normally seldom become ill from PCP, even though the pathogen that causes the disease is quite common in the environment.

Immunodeficiency disorders are classified as primary or secondary (the latter is also known as ...