The immune system is the body's network of cells and organs that is specially developed for fighting off infections. Weaknesses in the immune system can be either congenital, meaning someone is born with them, or acquired, caused by outside factors. A person with decreased immunity is at an increased risk for infections and illnesses.
The immune system is very complex, and contains numerous types of cells and proteins that function to fight off infections and keep the body healthy. A defect or mutation in any one of these components can lead to a congenital immunodeficiency, also referred to as a primary immunodeficiency. The most common defect is IgA deficiency, which can happen as often as 1 in 333 people, and is often asymptomatic; however, patients can also experience frequent sinus, urinary tract and intestinal infections. The other extreme of primary immunodeficiency is severe combined immunodeficiency, or SCID, which affects 1 in 50,000 people. In this condition, there is a problem with the body generating T cells, and mature T cells do not develop. This leads to severe life-threatening infections, especially from viruses and fungi. Babies born with SCID require a bone marrow transplant for survival. Hundreds of other primary immunodeficiencies have been described that fall somewhere in between these two in terms of clinical severity, and many more have yet to be discovered.
Immunodeficiency Due to Viruses
Human Immunodeficiency Virus, or HIV, was characterized in the early 1980s. HIV is a virus that infects human T helper cells and leads to an inability to fight off viruses and certain parasites. As of 2006, one in 5000 people in the U.S. were infected with the virus, while in some African countries, the prevalence was roughly one in five people. The treatment for HIV is combination therapy with highly active antiretroviral therapy, or HAART, which typically includes three different drugs. This drug regimen has greatly improved long-term survival for patients with HIV and slows the progression to AIDS, acquired immunodeficiency syndrome. Another virus that can cause severe acquired immunodeficiency is human T-lymphotrophic virus, or HTLV, which is most common in Japan and other parts of Asia. Other viruses like Epstein-Barr virus, which causes mononucleosis, and cytomegalovirus can interfere with the body's normal immunity, but the impact is typically not as severe.
Immunodeficiency Due to Malnutrition
Deficiency in certain vitamins and minerals can lead to decreased immune system function. Vitamins A, E and C, along with zinc, copper, iron and selenium, have all been shown to be important for either neutrophil, T cell, or antibody function. People with a poorly balanced diet and those with intestinal disease causing decreased absorption of nutrients are at risk.
Immunodeficiency Due to Other Diseases
Women affected with Turner's Syndrome can have low antibody levels, poor T cell function and problems with neutrophil-mediated killing of bacteria. Down's syndrome can cause similar problems. People who suffer from sickle cell disease invariably lose the function of their spleen over time, and this puts them at risk for streptococcal pneumonia and salmonella bone infections. Cystic fibrosis patients have difficulty clearing mucous from their lungs, leading to a high risk of pneumonia, especially with Pseudomonas organisms.
Other Factors Causing Low Immunity
Stress has been shown to modulate the function of immunologic signaling molecules, but this effect varies greatly from one person to another. Aging also appears to have a negative impact on immunity. Exposure to extreme environmental conditions, such as space flight, high altitudes and ionizing radiation, can impair the body's normal ability to fight off infections as well.