The immune system protects the body from potentially harmful substances (antigens) such as microorganisms, toxins, cancer cells, and blood or tissues from another person.

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

• Poor response to treatment
• Delayed or incomplete recovery from illness
• The presence of certain types of cancers (such as Kaposi's sarcoma or non-Hodgkins lymphoma)
• Certain opportunistic infections [such as Pneumocystis carinii pneumonia (PCP) or recurrent fungal/yeast infections]

• White blood cell count, antibody/immunoglobulin levels
• T (thymus derived) lymphocyte count
• Complement levels or other measurements of immune response components (decreased levels)

Usually, the immune response is a desired effect. Even deliberate immunosuppression (in the treatment of autoimmune disorders for example) attempts to maintain a balance between suppression of parts of the immune system and the ability to fight disease and infection.

The goal of treatment for immunodeficiency disorders includes protection against (and treatment of) diseases and infections.

Avoid contact with persons who have any infection or contagious disorder. This includes avoiding contact with people who have been immunized (vaccinated) within the past two weeks.

Any illness/infection is treated aggressively. This may involve prolonged use of antimicrobials (antibiotics, antifungal medications, etc.), use of powerful antimicrobials to treat any infection, and preventive (prophylactic) treatments.

Interferon (used to treat viral infections and some types of cancer) and Zidovudine (AZT, used to treat AIDS) are two immunostimulant drugs (medications that increase the efficiency of the immune system).

Persons with HIV and AIDS may take combinations of drugs to reduce the amount of virus in their immune systems, thus improving their immunity.

Patients undergoing a planned splenectomy should be vaccinated 2 weeks prior to the surgery against encapsulated organisms, such as Streptococcal pneumonia.

Bone marrow transplant may be used to treat certain immunodeficiency conditions.

Passive immunity (administration of antibodies produced by another person or animal) may occasionally be recommended to prevent illness after exposure to a microorganism.

• Disease development
• Frequent or persistent illness
• Opportunistic infections
• Increased risk for certain cancers/tumors

Call your health care provider immediately if you are taking immunosuppressive drugs (e.g., chemotherapy or prednisone), and you develop a fever greater than 100.5F, or have cough with shortness of breath. If a stiff neck and headache accompany your fever, go directly to an emergency department.

Contact your health care provider if you have recurrent yeast infections or oral thrush.