When part of the immune system is either absent or its function is hampered, an immune deficiency disease may result. An immune deficiency disease may be caused either by an intrinsic (inborn) defect in the cells of the immune system or an extrinsic environmental factor or agent may damage the immune system. In the former case, the immune deficiency disease is a primary immune deficiency disease. When the damage is caused by an extrinsic force, such as an environmental factor or agent, it is called a secondary immune deficiency disease.
For example, AIDS is a secondary immune deficiency disease caused by the HIV virus. The Primary Immunodeficiency Diseases are a group of disorders caused by basic defects in immune function that are intrinsic to, or inherent in, the cells and tissues of the immune system. There are nearly 100 primary immune deficiency diseases. Some are relatively common, while others are relatively rare. Although there are some that affect a single cell or protein of the immune system, others may affect more than one component of the immune system. Although primary immune deficiency diseases may differ from one another in many ways, they share one important feature. They all result from a defect in one of the functions of the normal immune system.
The primary immune deficiencies result from defects in T-lymphocytes, Blymphocytes, phagocytic cells or the complement system. Most of them are inherited diseases and may run in families, such as X-linked agammaglobulinemia (XLA) or Severe Combined Immunodeficiency (SCID). Other primary immune deficiencies, such as Common Variable Immunodeficiency (CVID) and Selective IgA Deficiency do not appear to be inherited in a clear cut or predictable fashion. In these disorders the cause is unknown but the interaction of genetic and environmental factors may play a role in their causation. Because one of the most important functions of the normal immune system is to protect us against infection, patients with primary immune deficiency diseases commonly have an increased susceptibility to infection. This increased susceptibility to infection may include too many infections, infections that are difficult to clear up, or unusually severe infections. The infections may be located in the sinuses (sinusitis), the bronchi (bronchitis), the lung (pneumonia) or the intestinal tract (infectious diarrhea). Another function of the immune system is to discriminate between the individual ("self") and foreign material (“non-self”), such as microorganisms, pollen or even a transplanted kidney from another individual. In some immune deficiency diseases, the immune system is unable to discriminate between "self" and (non-self). Therefore, in addition to an increased susceptibility to infection, patients with immune deficiencies may have autoimmune diseases in which their immune system attacks their own cells or tissues as if they were foreign or “non-self”. Primary immune deficiency diseases can occur in individuals of any age. The original descriptions of these diseases were in children, but as medical experience has grown many adolescents and adults have been diagnosed with primary immune deficiency diseases. This is in part due to the fact that some of the disorders, such as Common Variable Immunodeficiency Disease and Selective IgA Deficiency, may have their initial clinical presentation in adult life. But it is also due to the fact that effective therapy exists for nearly all of the disorders and patients diagnosed in infancy and childhood now reach adult life as productive members of society.