Immune System Diseases

The immune system usually protects you from pathogens and keeps you well. However, like any other body system, the immune system can malfunction or become diseased. Sometimes the immune system responds to harmless foreign substances as though they were pathogens. Sometimes it mistakes self for nonself and launches an attack against the body’s own cells. Certain diseases can also attack and damage the immune system so it loses the ability to defend the body.

Allergies

An allergy is a disease in which the immune system makes an inflammatory response to a harmless antigen. Any antigen that causes an allergic reaction is called an allergen. You can be exposed to allergens by inhaling or ingesting them or by having direct skin contact with them.

Allergies can vary greatly from person to person. Some people are allergic to many allergens, others to few or none. A tendency to develop allergies can be inherited, so if your mom or dad has allergies, you are more likely to have them as well. Allergy symptoms may be mild or severe. They may develop immediately after exposure to an allergen or not until several days after exposure.

Severity of Allergies

Allergy symptoms are caused by the release of histamines, the chemicals that also stimulate inflammation. The symptoms range from scarcely noticeable to potentially fatal. Typical symptoms of mild allergies include itchy eyes, sneezing, and skin rashes. These symptoms may be uncomfortable, but they are not life threatening. Mild allergy symptoms are often treated with antihistamines. Antihistamines are drugs that reduce or eliminate the effects of histamines.

Immunotherapy, commonly called “allergy shots,” is sometimes recommended for more severe allergies. A person with an allergy is injected with larger and larger amounts of the offending allergen over a period of months or years. This gradually desensitizes the person’s immune system to the allergen. Rather than just treating the symptoms of the allergy, immunotherapy reduces the severity of the allergy or eliminates the allergy altogether.

The most severe allergic reaction is anaphylaxis. Anaphylaxis is an allergic response in which there is a sudden, massive release of histamines throughout the body. This causes collapse of the circulatory system and severe constriction of the breathing passages. Without emergency treatment, anaphylaxis is likely to be fatal. Treatment is usually injection of epinephrine. Epinephrine is the “fight-or-flight” hormone that your adrenal glands normally produce when you are in danger. The hormone suppresses non-emergency body processes, including the immune response.

Immediate Hypersensitivity Reaction

When exposure to an antigen causes immediate allergy symptoms, the response is called an immediate hypersensitivity reaction. This is a humoral immune response. Examples of allergens that cause this type of reaction include pollens, bee stings, and peanuts. Anaphylaxis may occur if the allergy is severe.

Allergic rhinitis is a common immediate hypersensitivity reaction. It affects mainly mucous membranes lining the nose. Typical symptoms include runny nose and nasal congestion. Pollens are the most common cause of allergic rhinitis. Tiny pollens of wind-pollinated plants like ragweed (Figure 1) are the usual culprits. Other causes of allergic rhinitis include mold, animal dander, and dust. Allergic rhinitis may occur seasonally or year-round, depending on its cause.

Figure 1: Ragweed, a common cause of allergic rhinitis.

Allergic rhinitis is often called hay fever, although pollen—not hay—is the most likely cause. It is called hay fever because it is most common during the time of year when hay is cut. This is also the time of year when plant pollens are most concentrated in outdoor air.

Delayed Hypersensitivity Reaction

When an antigen causes allergy symptoms hours or days after exposure, the response is called a delayed hypersensitivity reaction. This is a cell-mediated immune response. Examples of allergens that cause delayed hypersensitivity reactions include poison ivy, poison oak, and poison sumac. If you have skin contact with these plants and are allergic to them, a rash, like the one in Figure 2, may develop.

Figure 2: Allergic rash caused by contact with poison ivy.

Autoimmune Diseases

Autoimmune diseases occur when the immune system fails to recognize the body’s own molecules as self and attacks the body’s cells as though they were foreign invaders. Relatively common autoimmune diseases include rheumatoid arthritis, type 1 diabetes mellitus, multiple sclerosis, and systemic lupus erythematosus (Table 1). These four diseases are described in the table below. They are currently incurable, but treatment can help relieve the symptoms and prevent some of the long-term damage.

The causes of autoimmune diseases are not known for certain. One way autoimmunity may develop is through “molecular mimicry.” This occurs when a person is infected with pathogens bearing antigens similar to the person’s own molecules. When the immune system mounts an attack against the pathogens, it also attacks body cells with the similar molecules. Some people inherit genes that increase their risk for an autoimmune disease. Female sex hormones may also increase the risk. This may explain why autoimmune diseases are more common in females than males and why they usually begin after puberty.

Immunodeficiency Diseases

Immunodeficiency occurs when one or more components of the immune system are not working normally. As a result, the ability of the immune system to respond to pathogens and other threats is decreased. A person with immunodeficiency may suffer from frequent, lifethreatening infections. In other words, an individual with a compromised immune system (for example, a person with AIDS) may be unable to fight off and survive infections by microorganisms that are usually benign. Immunodeficiency can be present at birth or acquired after birth.

Congenital Immunodeficiency

Congenital immunodeficiency is present at birth and usually caused by a genetic disorder. Such disorders are relatively rare. For example, thymic aplasia—a genetic disorder characterized by an absent or abnormal thymus—occurs in about 1 out of 4,000 births. People with thymic aplasia are unable to produce normal T cells. They have frequent infections and increased risk of autoimmune diseases.

Acquired Immunodeficiency

Acquired immunodeficiency occurs when immune function declines in a person who was born with a normal immune system. There are many possible causes for declining immune function. Age is one cause. The immune system naturally becomes less effective as we get older, starting in middle adulthood. This helps explain why older people are more susceptible to disease. Other possible causes of declining immune function include obesity, alcoholism, and illegal drug abuse. In developing countries, malnutrition is a common cause.

Many medications can interfere with normal immune function and cause immunodeficiency. Immune suppressive drugs are deliberately given to people with autoimmune diseases and transplanted organs. Many other drugs have immune suppression as a side effect. Chemotherapy drugs for cancer are especially likely to suppress the immune system.

Several kinds of cancer attack cells of the immune system and cause immunodeficiency. For example, in chronic lymphatic leukemia, abnormal B cells that can’t fight infection grow out of control and crowd out healthy B cells. Certain pathogens can also attack cells of the immune system. In fact, the virus known as HIV is the most common cause of immunodeficiency in the world today.

HIV and AIDS

HIV, or human immunodeficiency virus, is the virus that causes AIDS. AIDS stands for acquired immune deficiency syndrome. It is a late stage in the progression of an HIV infection.

HIV Transmission

HIV is transmitted, or spread, through direct contact of mucous membranes or the bloodstream with a body fluid containing HIV. Body fluids that can contain HIV include blood, semen, vaginal fluid, preseminal fluid, and breast milk. Transmission of the virus can occur through sexual contact or use of contaminated hypodermic needles. HIV can also be transmitted through a mother’s blood to her baby during late pregnancy or birth or through breast milk after birth. In the past, HIV was transmitted through blood transfusions. Because donated blood is now screened for HIV, the virus is no longer transmitted this way.

HIV and the Immune System

HIV destroys helper T cells. Recall that helper T cells are needed for normal humoral and cell-mediated immunity. When HIV enters a person’s bloodstream, proteins on the coat of the virus allow it to fuse with the host’s helper T cells. The virus injects its own DNA into the host’s helper T cells and uses the T cells’ “machinery” to make copies of itself. The copies of the virus bud off from the host’s cells, destroying the cells in the process. Copies of the virus go on to infect other helper T cells throughout the body.

During the first several weeks after HIV infection, the immune system tries to fight off the virus. As shown in Figure 3, the initial immune response temporarily reduces the number of virus copies in the blood. However, the immune system is unable to destroy the virus, and it continues to multiply in the lymphatic system. How is HIV able to evade the immune system? There are at least two ways:

Figure 3: Average numbers of helper T cells and HIV copies in untreated HIV infections.

• The virus undergoes frequent mutations that keep changing the antigens on its coat. This prevents antigen-specific lymphocytes from developing that could destroy the virus.
• The virus uses the host’s cell membranes to form is own coat. This covers up viral antigens so they cannot be detected by the host’s immune system.

Over the next several years, helper T cells continuously decline in the blood, while copies of the virus keep increasing. As the number of helper T cells declines, so does the ability of the immune system to make an immune response. The HIV-infected person starts showing symptoms of a failing immune system, such as frequent infections.

Treatment with antiviral medications can slow down the increase in virus copies, although they do not eliminate the virus altogether. The medications usually lengthen the time between infection with HIV and the development of symptoms. However, currently there is no cure for HIV infection or AIDS and no vaccine to prevent infection, although this is a field of intense study by biomedical scientists.

AIDS

AIDS is not a single disease but a collection of symptoms and diseases. It is the result of years of damage to the immune system by HIV. AIDS is diagnosed when helper T cells fall to a very low level and the infected person develops one or more opportunistic diseases.

Opportunistic diseases are infections and tumors that are rare in people with a healthy immune system but common in immunodeficient people. Opportunistic diseases include pneumocystis pneumonia and Kaposi’s sarcoma, a type of cancer. The diseases are called opportunistic because they take advantage of the “opportunity” to infect a person with a damaged immune system that can’t fight back. Opportunistic diseases are often the direct cause of death of people with AIDS.

AIDS was first identified in 1981. Since then it has killed more than 25 million people worldwide, many of them children. The hardest hit region is sub-Saharan Africa, where antiviral medications are least available. The worldwide economic toll of AIDS is also enormous.