Immune Deficiency Disease

I

Introduction

Immune Deficiency Disease, any one of a group of diseases caused by the impairment of the immune system. People with an immune deficiency disease typically become dangerously ill from the invasion of microorganisms that do not seriously harm healthy people. Immune deficiency diseases are characterized by frequent, often life-threatening infections, most commonly in the respiratory system, ear, or intestines. Other common problems are organ or bone infections, blood infections, and meningitis (infections of the brain membranes).

II

How Immune Deficiencies Develop

The immune system is composed of a collection of cells, specialized organs, and proteins in the blood that work together to defend against foreign substances, which enter the body from the external environment. A large number of genes are required to create the components of the immune system. Some immune deficiency diseases arise when one or more of these genes is defective. Genetic immune deficiency leads to frequent bacterial, viral, or fungal infections. It can increase the likelihood of cancer, or diseases like rheumatoid arthritis and the kind of anemia that results from premature destruction of red blood cells.

The immune system may be impaired during fetal development, resulting in a congenital immune deficiency disorder, one that is present from birth but not necessarily inherited. There are nearly 100 inherited or congenital immune deficiency diseases collectively called primary immune deficiency diseases, and these disorders develop in 1 in every 10,000 people.

Immune deficiency disease can also develop as a result of an illness, traumatic injury, or therapeutic drug that damages the functioning of the immune system. More common than primary immune deficiency, this type of immune deficiency is called secondary immune deficiency disease. Infectious viruses such as German measles, or rubella, measles, Epstein-Barr virus, and human immunodeficiency virus (HIV) can damage the immune system; so can drugs that suppress the immune system, a lack of proper nutrition, and the process of aging. The progress of acquired immune deficiency disease can be reversed if the underlying cause is treatable.

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III

Primary Immune Deficiency Diseases

One part of the immune system fights a variety of invading organisms using special cells called lymphocytes. B lymphocytes fight infection by producing proteins called antibodies, which travel through the body to attack specific microbes, particularly bacteria. T lymphocytes trigger a variety of defenses to combat viruses, fungi, and some cancer cells. Primary immune deficiencies can impair either B lymphocytes, T lymphocytes, or sometimes both.

Deficiencies in B-lymphocyte production interfere with the production of certain types of antibodies known as immunoglobulins. The complete absence of the immunoglobulin gamma globulin is called agammaglobulinemia. One form of this serious disease typically develops in male infants under the age of two. These infants are susceptible to recurrent lung and sinus infections.

The most common and mildest immune deficiency disease is selective IgA deficiency, which results from a lack of the disease-fighting antibody immunoglobulin A (IgA). Occurring in as many as 1 in 400 people, IgA deficiency may present no symptoms at all in some people while others may have an increased number of respiratory or gastrointestinal infections.

Immune deficiencies of T lymphocytes prevent the body from fighting virus and fungal infections. People deficient in T lymphocytes typically develop persistent fungal infections affecting the skin, mouth, and vagina. Another T-lymphocyte deficiency is DiGeorge Syndrome, a disease characterized by abnormal development of the human embryo and fetus that leads to improper development of the fetus’s thymus, the gland situated behind the breastbone in the chest. This condition results in a deficiency in the number of T lymphocytes, specialized cells that develop under the influence of the thymus. Infants with DiGeorge Syndrome typically have characteristic facial deformities, heart disorders, and a high susceptibility to infection.

Some immune deficiency diseases result from both B- and T-lymphocyte impairment, such as the inherited disorder known as Wiskott-Aldrich Syndrome. Affecting only male infants, Wiskott-Aldrich commonly causes skin problems like eczema, persistent and bloody diarrhea, and recurrent respiratory infections.

The most dangerous group of immune deficiencies resulting from B- and T-lymphocyte malfunction is severe combined immunodeficiency (SCID). Within 3 to 12 months after birth, infants with SCID develop recurrent bacterial, viral, or fungal infections. About 30 percent of these infants suffer from an enzyme deficiency known as adenosine deaminase (ADA) deficiency. In the absence of adenosine deaminase, T lymphocytes cannot develop normally.

Pneumonia is particularly common among infants with SCID, as are meningitis and infections of the circulatory system. Even a relatively mild virus like herpes simplex, which causes the common cold sore, can prove fatal to such infants. If left untreated, infants with SCID may die by the age of two.

Another type of primary immune deficiency involves members of the complement system, a set of proteins and enzymes that regulate the activities of antibodies in the blood. The complement system acts as an early warning mechanism by alerting other parts of the immune system to the fact that disease-causing agents have invaded the body. A weak complement system leads to recurrent infections, meningitis, or excessive swelling of the limbs or face that resembles an allergic reaction.

Phagocytic deficiency is a major cause of disease in newborns. Phagocytes are cells that surround and destroy disease-causing microorganisms, thereby keeping infections from arising. In some cases, such cells may not be present in normal numbers due to underproduction or premature destruction. Alternately, the phagocytes may be present in sufficient quantities but may not function properly.

Primary immune deficiencies caused by genetic factors have been studied extensively, and a number of genes responsible for these defects have been identified. Some genes have been found on the X chromosome, the sex chromosome inherited from the mother. These X-linked diseases include x-linked agammaglobulinemia, Wiskott-Aldrich Syndrome, and some forms of SCID.

IV

Secondary Immune Deficiencies

Previously healthy individuals who develop certain illnesses or undergo surgery or drug therapy may develop impaired immune systems. Trauma from severe burns or alcoholism that damages the liver can result in immune deficiency. Drugs used to suppress the immune system such as corticosteroids, those used before an organ transplantation, or radiation therapy, may also hurt the immune system.

Infections that cause immune deficiency include acquired immunodeficiency syndrome (AIDS), caused by the human immunodeficiency virus (HIV). AIDS causes a progressive depletion of T lymphocytes so that there are not enough to fight infections. Low T-lymphocyte levels also make the body vulnerable to a host of infections that would not develop in a person with a healthy immune system. These opportunistic infections include Pneumocystis carinii, a type of pneumonia, toxoplasmosis, and fungal infections.

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