The immune system is an intricate network of specialized cells and organs that defends your body against attacks by infectious microorganisms such as bacte- ria, viruses, and fungi, and by larger invaders such as worms. The human body provides an ideal habitat for many microorganisms, both beneﬁcial and harmful. When a harmful microorganism tries to enter the body, the immune system will attempt to block its entry. If that fails, the immune system will search out and destroy the invader. The immune system also has a role in allergies, autoimmune diseases, and controlling cancer.
The immune system has several remarkable characteristics. When working properly, it can distinguish between the body’s own cells and those that come from outside of the body. It also can remember previous exposures to certain microorganisms. For example, if you have had chickenpox, you usually will not develop it again because your immune system knows that you have already been exposed and will respond even more aggressively to that virus. Not only can the immune system recognize millions of different invading microorganisms, it also can produce speciﬁc molecules to ﬁght each one. The success of the immune system depends on an elaborate system of communication and checks and bal- ances that quickly pass information back and forth along pathways.
Bone marrow (the soft tissue in the hollow shafts of the long bones) is a key component of your immune system. It produces white blood cells called lym- phocytes, which are essential components of the immune system. Another important part of the immune system is the spleen (a ﬁst-sized organ in your upper left abdomen). The spleen produces lymphocytes and phagocytes (another type of white blood cell). Other organs of the immune system include the thymus gland (a gland in the upper chest), tonsils (masses of lymph tissue at the back of the throat), adenoids (swellings of lymph tissue at the back of the roof of the
The Immune System
mouth), appendix (a small, ﬁngerlike projection from the large intestine), and lymph nodes. The lymph nodes are bean-shaped organs in the head, neck, chest, abdomen, and groin. They ﬁlter a watery ﬂuid called lymph, which bathes the body tissues and contains lymphocytes.
Once a foreign cell or infectious microorganism is detected, your body begins an immune response to defend itself. Several different types of white blood cells travel to the site of infection to stop the invader from going any farther. White blood cells called phagocytes surround the invading microorganisms and destroy them, dying in the process. Other white blood cells, called mast cells, release chemicals at the site of the infection to trigger the swelling and redness that are characteristic of inﬂammation.
If the invading microorganism survives, it is subjected to a second attack. The body sends more sophisticated white blood cells, known as B lymphocytes and T lymphocytes (also called B cells and T cells), to the site of the infection.
B lymphocytes can recognize speciﬁc microorganisms and produce proteins called antibodies, which are specially designed to destroy these microorganisms. Some B lymphocytes will become “memory cells,” which will recognize the invading microorganism if it tries to enter the body again and will automatically produce antibodies to destroy it.
There are two types of T lymphocytes—killer cells and helper cells. Killer T lymphocytes destroy microorganisms that have infected the cells of the body by attaching to the invading microorganisms and releasing chemicals that destroy them. Killer T lymphocytes also rid the body of cancer cells. Helper T lymphocytes protect the body from both invading microorganisms and cancer cells by enhancing the actions of the killer T lymphocytes and by controlling var- ious aspects of the immune response.
Some people may lack one or more components of the immune system, or a component may not function properly. This is called an immunodeﬁciency dis- order, which means that the person cannot adequately ﬁght off infection. In other people, a speciﬁc substance such as ragweed pollen or saliva on cat dander (ﬂakes of dead skin), which is harmless to most people, can provoke an inappro- priate immune response. This is commonly called an allergy. In some other peo- ple the immune system mistakenly attacks the body’s own cells and tissue, resulting in what is known as an autoimmune disease. One such disease is type 1 diabetes (see page 365).
A weakened immune system can be congenital (present from birth), or it may result from an inherited disorder or an infection. It also may occur as a side effect of drug treatment. People with weakened immune systems have difﬁculty ﬁght- ing infections that are easily handled by a healthy immune system. When a per- son’s immune system is ineffective, harmful microorganisms can thrive and multiply rapidly, causing life-threatening infections.
A small number of infants are born with defects in their immune systems. Infants born with defective B lymphocytes are unable to produce antibodies and therefore are vulnerable to a wide variety of infections. Doctors treat this type of disorder with injections of live antibodies. Some infants lack T lymphocytes because they were born without a thymus gland or because the thymus gland is small and abnormal. This disorder requires a thymus gland transplant. In very rare cases, infants are born without an immune system, a condition known as severe combined immunodeﬁciency disease. Children with this disease gained notoriety by living for years in germ-free rooms or “bubbles.” A few of these children have been successfully treated with bone marrow transplants.
The best-known immunodeﬁciency disorder is acquired immunodeﬁciency syndrome (AIDS), which is caused by the human immunodeﬁciency virus (HIV). AIDS often leads to various otherwise rare diseases, such as Pneumo-cystis carinii pneumonia and Kaposi’s sarcoma (a form of cancer). These dis- eases are called opportunistic diseases because they rarely occur in people with healthy immune systems, but occur more in people with weakened immune sys- tems, in whom conditions for development are favorable. HIV infection also damages the central nervous system (the brain and the spinal cord), which leads to dementia (progressive deterioration of mental functioning).
HIV infection may cause no symptoms in the early stages or can produce ﬂu- like symptoms, including fever, chills, fatigue, sweating at night, and a dry cough. Other symptoms include sudden unexplained weight loss and chronic diarrhea. Although some infected people may have no symptoms, they still can transmit the virus to others.
HIV is transmitted by sexual contact or by direct contact with infected blood or body ﬂuids. HIV can be transmitted from an infected woman to a fetus during pregnancy or to an infant during childbirth or through breast milk. HIV also can be transmitted by sharing needles during intravenous drug use. In the past, some people were infected with HIV through blood transfusions or contaminated blood products. Today, however, all donated blood is routinely screened for HIV, and the blood supply in the United States is safe.
At present there is no cure for AIDS. Early diagnosis and treatment are vital. Certain medications that are currently available, when used in combination, seem to slow development of the disease. These medications include nucleoside analog reverse transcriptase inhibitors (such as zidovudine, also called AZT), which interrupt an early stage of virus replication, and protease inhibitors (such as indinavir, ritonavir, and saquinavir), which interrupt the same process at a later stage. Although these “cocktail” drug regimens typically cause unpleasant side effects such as nausea and diarrhea, when started early, they are enabling many people infected with HIV to live longer, more productive lives. Other drugs designed to combat the virus, bolster the immune system, and prevent or treat opportunistic infections that result from HIV infection are now being tested. Research on an AIDS vaccine is also under way. For more information on HIV and AIDS, see page 186.
Immunodeﬁciency can be an unwanted side effect of certain medications, such as those used to treat cancer. Anticancer drugs used during chemotherapy affect cells that divide rapidly, including the white blood cells (lymphocytes) that ﬁght infection. Because of this, people undergoing chemotherapy may become more susceptible to opportunistic infections.
An allergy is an exaggerated or inappropriate response of the immune system to a substance that is harmless to most people. Substances that can cause such reac- tions are called allergens. Common allergens include pollen, dust particles, cer- tain foods, insect venom, mold, and medications. Doctors think that the reason
some people are allergic to a particular substance is because they have inherited a tendency to be allergic (although not necessarily to that particular substance). Being exposed to a potential allergen when the body’s defenses are lowered or weakened—such as during a viral infection—seems to contribute to the devel- opment of an allergy. People with allergies are often sensitive to more than one allergen.
During an allergic reaction, the immune system is responding to a false alarm. When a person comes into contact with an allergen, the immune system launches an inappropriate immune response by releasing large amounts of an antibody (a disease-ﬁghting protein) called immunoglobulin E. Each time the person encounters an allergen, these antibodies signal the body to produce pow- erful chemicals, such as histamine, that travel to where the allergen is located (such as the airways, the skin, or the surface of the eye) and cause inﬂammation. Symptoms of inﬂammation include redness, swelling, pain, and warmth. When pollen, dust, or other airborne allergens cause inﬂammation of the mucous mem- brane that lines the nose, the condition is known as allergic rhinitis. Common symptoms include runny nose, coughing, nasal congestion, and sneezing.
One of the most common allergies is pollen allergy. Each spring, summer, and fall, trees, weeds, and grasses release tiny particles called pollen, which ride on currents of air. The pollen enters the nose and throat, triggering allergic reac- tions in susceptible people. Among North American plants, weeds, especially ragweed, are the most proliﬁc producers of pollen. In fact, the popular term “hay fever” typically refers to allergy to ragweed pollen. Other important pollen-producing weeds include sagebrush, redroot pigweed, tumbleweed, and English plantain. Trees that produce allergic pollen are oak, ash, elm, hickory, pecan, box elder, and mountain cedar. Although a wide variety of grasses grow in North America, only about seven types produce pollen that causes allergic responses.
Each plant has a pollinating period that is the same from year to year. The rel- ative lengths of night and day seem to be the trigger for pollination, so the farther north you live, the later (and shorter) the pollinating period and the later (and shorter) the allergy season. A pollen count measures how much pollen is in the air at a given time. Pollen counts tend to be highest early in the morning on warm, dry, breezy days and lowest on chilly, wet days. If you have allergic rhini- tis, try to stay indoors as much as possible on high-pollen-count days, especially in the morning. If you work outdoors, wear a face mask that ﬁlters out pollen. If possible, take your vacation at the height of pollen season and go to a location, such as the beach, where pollen counts are minimal.
House dust probably is the most common cause of airborne allergies, produc- ing symptoms of allergic rhinitis. House dust is a mixture of potential allergens. It often contains fabric ﬁbers, cotton lint, feathers and other stufﬁng materials, bacteria, mold, food particles, bits of plants and insects, and dust mites (micro-
scopic insects). Dust mites live in bedding, upholstered furniture, and carpets. They thrive in summer, and they can live all year in warm, humid houses. The mites produce waste-containing proteins, which are the actual causes of the allergic reaction. Waste products of cockroaches also are are an important cause of allergy symptoms in infested households, especially in urban areas.
If you have a dust allergy, take the time to dustproof your home, especially your bedroom. Eliminate as many dust-gathering and dust-producing items— such as wall-to-wall carpeting, blinds, down-ﬁlled blankets, feather pillows, forced-air heating vents, pets, and closets full of clothing—as you can. Encase your mattress in a zippered, plastic, dustproof cover. Wash all of your bedding once a week in water hotter than 130 degrees Fahrenheit. Frequently wipe with a damp cloth all surfaces where dust accumulates.
Many people are allergic to animals, particularly household pets. Pet allergy is not triggered by the dander or hair of dogs or cats but rather by proteins in their saliva, which they transmit to their hair through licking. An animal allergy can take 2 or more years to develop and may not subside until 6 or more months after contact with the animal has ended. The best way to avoid an allergic reaction is to stay away from animals—even if it means ﬁnding a new home for a beloved pet. To decrease the level of cat allergens, try bathing the cat weekly, which neu- tralizes the protein on the fur. And keep the animal out of the bedroom.
Mold is a type of fungus that grows seasonally and causes allergic reactions in some people. Mold levels peak in late summer but, in warmer climates, molds thrive all year and can cause allergic symptoms year-round. Molds can be found wherever there is moisture. Outside, they grow on rotting logs and decomposing leaves. In the house, molds thrive in basements, bathrooms, damp closets, refrig- erator drip trays, houseplants, air conditioners and humidiﬁers, garbage cans, mattresses, and old foam rubber pillows. When inhaled, tiny mold spores often evade the protective mechanisms of the nose and upper airways and reach the lungs. There they can cause bronchospasm (temporary narrowing of the airways in the lungs) or trigger an asthma attack (see page 245) in a person who has asthma.
To minimize your contact with allergy-triggering molds, have someone else mow your lawn and rake leaves, or wear a tight-ﬁtting mask when you do these chores. Clean and disinfect your bathroom ﬁxtures regularly, and be sure to air out closets, mattresses, pillows, and garbage cans. Regularly clean your air- conditioner ﬁlter and your humidiﬁer ﬁlter and water reservoir according to the manufacturer’s instructions. A dehumidiﬁer can help dry out your basement, but you must clean the machine frequently to prevent mold buildup.
In addition to sneezing, coughing, runny nose, and nasal congestion, people with allergic rhinitis also may have itchy, watery eyes and conjunctivitis (an inﬂammation of the lining of the eyelids). Dark circles typically appear under the eyes, caused by increased blood ﬂow near the sinuses. Some people
with allergies may develop asthma, a serious and potentially life-threatening respiratory disorder that causes tightness in the chest, wheezing, and sometimes severe shortness of breath.
Your health history (see page 82) is an important tool that can help your doc- tor diagnose a potential allergy. While taking your medical history, he or she can determine whether your symptoms recur at the same time each year, which sug- gests a seasonal allergen such as pollen. The doctor also will ask questions to determine what substances seem to trigger your allergies. To conﬁrm this infor- mation, the doctor may perform skin testing, in which tiny amounts of suspected allergens are applied to, injected under, or scratched onto the skin. A small, red- dened area will appear in the area where any substance to which you are allergic was applied. Blood tests also may be performed to detect blood levels of anti- bodies to a speciﬁc allergen.
Doctors generally recommend three approaches to the treatment of allergies: avoidance, medication, and allergy injections. For some people, complete avoid- ance of an allergen such as pollen or mold may require moving to a location where the allergen does not grow. Others may need to give up a favorite pet. If you have allergies, you may not be able to completely avoid the substances that provoke an allergic reaction. That is why doctors try to control allergic symp- toms with medications, sometimes in combination. The most common medica- tions prescribed for this purpose include antihistamines, which counter the effects of histamine, the inﬂammation-inducing chemical released by the body during an allergic reaction. Nasal corticosteroids, which are sprayed into the nose, combat inﬂammation, swelling, and mucous secretion. Another nasal spray, called cromolyn sodium, prevents allergic reactions from starting. Effec- tive antihistamines and decongestants also are available over the counter. Ask your doctor for recommendations.
Taking a series of allergy injections (immunotherapy) is the only method of reducing your allergy symptoms over the long term. This treatment usually is given over the course of 2 or 3 years. The doctor injects you with gradually increasing doses of the allergen. In response, your body decreases production of antibodies to that substance and begins producing protective antibodies instead. About 85 percent of people with allergic rhinitis experience a substan- tial reduction in their symptoms within 2 years of beginning their allergy injections.