The immune system is made up of lymphoid tissue in the body, which includes:
- Bone marrow
- Lymph nodes
- Parts of the spleen and gastrointestinal tract
Proteins and cells in the blood are also part of the immune system.
The immune system helps protect the body from harmful substances called antigens. Examples of antigens include bacteria, viruses, toxins, cancer cells, and foreign blood or tissues from another person or species.
When the immune system detects an antigen, it responds by producing proteins called antibodies that destroy the harmful substances. The immune system response also involves a process called phagocytosis. During this process, certain white blood cells swallow and destroy bacteria and other foreign substances. Proteins called "complements" help with this process.
Immune system disorders occur when the immune system does not fight tumors or harmful substances as it should. The immune response may be overactive or underactive.
Immunodeficiency disorders may affect any part of the immune system. Most commonly, these conditions occur when special white blood cells called T or B lymphocytes (or both) do not work as well as they should, or when your body doesn't produce enough antibodies.
Inherited immunodeficiency disorders that affect B cells include:
- Hypogammaglobulinemia, which usually leads to respiratory and gastrointestinal infections
- Agammaglobulinemia, which results in severe infections early in life, and is often deadly
Inherited immunodeficiency disorders that affect T cells may cause repeated Candida (yeast) infections. Inherited combined immunodeficiency affects both T cells and B cells. It may be deadly within the first year of life if it isn't treated early.
People are said to be immunosuppressed when they have an immunodeficiency disorder due to medicines that weaken the immune system (such as corticosteroids). Immunosuppression is also a common side effect of chemotherapy given to treat cancer.
Acquired immunodeficiency may be a complication of diseases such as HIV infection and malnutrition (especially if the person does not eat enough protein). Many cancers may also cause immunodeficiency.
People who have had their spleen removed have an acquired immunodeficiency, and are at higher risk for infection by certain bacteria that the spleen would normally help fight. Patients with diabetes are also at higher risk for certain infections.
As you get older, the immune system becomes less effective. Immune system tissues (especially lymphoid tissue such as the thymus) shrink, and the number and activity of white blood cells drop.
The following conditions and diseases can lead to an immunodeficiency disorder:
- Chediak-Higashi syndrome
- Combined immunodeficiency disease
- Complement deficiencies
- DiGeorge syndrome
- Job syndrome
- Leukocyte adhesion defects
- Bruton disease
- Congenital agammaglobulinemia
- Selective deficiency of IgA
- Wiscott-Aldrich syndrome