The immune system plays three important roles in the body:
To recognize the wide array of pathogens in the environment, the immune system relies on many cell types that operate together to generate immune responses. Those cells arise from stem cells in the bone marrow, they are found in lymphoid tissues throughout the body, and they circulate in the blood as white blood cells (WBCs). The main types of WBCs are granulocytes, monocytes, and lymphocytes. Each category has many specialized cell populations that are responsible for specific functions connected to the production of specific immune hormones (generically known as cytokines). Imbalances in these specialized populations or in their level of functional activity can result in inadequate or improper immune responses that may lead to pathologic outcomes. Diseases arising from immune dysfunction may be apparent immediately or observed only after an organism encounters an environmental challenge that causes immune cells to respond (such as an infection). Immune dysfunctions are in four major categories that need not be mutually exclusive: immune suppression, allergy, autoimmunity, and inflammatory dysfunction (inappropriate and/or misdirected inflammation). Although immune suppression usually is seen as an increased incidence of infections or an increased risk of cancer, allergic, autoimmune, and inflammatory disorders can be manifested as diseases affecting virtually any tissue. It is often difficult to diagnose such diseases, so they may or may not be medically categorized as immune disorders.
Suppression of immune responses can reduce resistance to infectious disease and increase the risk of cancer. Infection with the human immunodeficiency virus (HIV) is a well-recognized example of an acquired immune deficiency in which a specific type of lymphocyte (CD4+ T cells) is the target of the virus. The decline in the number of CD4+ T cells after HIV infection correlates with an increased incidence of infectious diseases, including fatal opportunistic infections, and with an increased incidence of several types of cancer. Treatment of cancer patients with toxic chemotherapeutic drugs suppresses the immune system by inhibiting the generation of new WBCs by the bone marrow and by blocking proliferation