The immune response consists of general actions such as phagocytosis, where white blood cells engulf and destroy "foreign" material. It protects against specific antigens by producing antibodies (immunoglobulins), which are molecules that attach to a specific antigen and make destruction of the antigen more efficient.

It also protects against specific antigens by producing lymphocytes (a group of white blood cells) that become specialized (sensitized). The sensitized lymphocytes "recognize" the foreign substance, and they destroy it.

Immunity is, in part, a product of lymphoid tissue in the body which includes the thymus, lymph nodes, tonsils, parts of the spleen and gastrointestinal tract, and bone marrow.

Lymphocytes (the specialized white blood cells that provide acquired immunity) are produced or mature in various lymphoid tissues. Lymphocytes are divided into two groups.

T lymphocytes become the sensitized lymphocytes that directly attack antigens (cellular immunity).

B lymphocytes produce antibodies (humoral immunity) that attach to the antigen and make phagocytes and body chemicals such as complement proteins much more efficient in the destruction of the antigen.

Immune system disorders occur when the immune response is inappropriate, excessive, or lacking. Immunodeficiency disorders occur when the immune system fails to fight tumors or invading substances.

This causes persistent or recurrent infections, severe infections by organisms that are normally mild, incomplete recovery from illness or poor response to treatment, and an increased incidence of cancer and other tumors. Opportunistic infections are widespread infections by microorganisms that are usually controllable.

This deficiency may affect any part of the immune system. Most commonly, it involves decreased functioning of T or B lymphocytes (or both), or deficient antibody production. The causes include congenital/inherited defects and acquired immunodeficiency caused by a disease that affects the immune system.

Examples of congenital immunodeficiency disorders of antibody production (B lymphocyte abnormalities) include hypogammaglobulinemia (lack of one or more specific antibodies) -- which usually causes repeated mild respiratory infections, and agammaglobulinemia (lack of all or most antibody production) -- which results in frequent severe infections and is often fatal.

Congenital disorders affecting the T lymphocytes may cause increased susceptibility to fungi, resulting in repeated Candida (yeast) infections. Inherited combined immunodeficiency affects both T lymphocytes and B lymphocytes. It is often fatal within the first year of life because there is no resistance to disease or infection.

People are said to be "immunosuppressed" when they experience immunodeficiency that is caused by medications such as corticosteroids or other immunosuppressant  medications. This is a desired part of treatment for disorders such as autoimmune disorders. It is used after organ transplantation to prevent transplant rejection.

Immunosuppression is also a common side-effect of chemotherapy to treat many types of cancer because the chemotherapy often reduces the number of white blood cells available to fight infection.

Acquired immunodeficiency may be a complication of diseases such as HIV infection and AIDS (acquired immunodeficiency syndrome). Malnutrition, particularly with lack of protein, can cause acquired immunodeficiency. Many cancers can cause immunodeficiency.

Those who have had a splenectomy, or removal of the spleen, face a higher risk of infection from certain encapsulated bacteria which the spleen would normally help fight.

Increasing age also reduces the effectiveness of the immune system. Immune system tissues (particularly lymphoid tissue such as the thymus) shrink with aging. There is also reduced lymphocyte number and activity with increasing age.

The following list contains conditions and diseases that can result in an immunodeficient state:

• Ataxia-telangiectasia
• DiGeorge syndrome
• Chediak-Higashi syndrome
• Job syndrome
• Leukocyte adhesion defects
• Panhypogammaglobulinemia
  • Bruton disease
  • Congenital agammaglobulinemia
• Selective deficiency of IgA
• Combined immunodeficiency disease
• Wiscott-Aldrich syndrome
• Complement deficiencies

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