A second line of defense is the intricate and complicated immune system * , whose function is to recognize and destroy foreign substances and microorganisms that enter the body. The properly functioning human immune system can distinguish between the body's own tissues and foreign substances. It recognizes foreign entities by the presence of antigens *
When bacteria, toxins, burns, or other culprits damage tissue, the injured tissues leak chemicals, including histamine * and other substances. This chemical mixture causes blood vessels around the damaged area to saturate injured tissues and make them swell. The increased flow of blood and fluid to the area also brings phagocytes * and other infection-fighting cells to take care of any toxins or other antigens in the area. Pus, which is a whitish-yellow fluid containing dead body cells and tissue, dead bacteria, dead toxins, and dead and living phagocytes, sometimes forms at the site of inflammation.
Lymphocytes (LIM-fo-sites), a kind of white blood cell that develops in bone marrow * , circulate throughout the body in the bloodstream and in the lymphatic system * . Lymphocytes are the part of the immune system that has the capacity to recognize and remember disease-causing agents.
Lymphocytes can be divided into two classes: B lymphocytes and T lymphocytes. B lymphocytes (or B cells) produce immunoglobulins (ih-myoo-no-GLAH-byoo-lins), also called antibodies. A specific B lymphocyte recognizes a specific antigen; its activity is triggered when it encounters its matching antigen. The activated B lymphocytes go to work, synthesizing and rapidly releasing large numbers of antibodies. The antibodies bind to specific antigens, and may in the act of binding neutralize the offending agent's capacity to do harm. Antibodies also stimulate the activity of phagocytic cells, which work with another type of white blood cell called phagocytes—scavenger cells that surround and digest infected cells or microorganisms—and destroy the invaders.
T lymphocytes (or T cells) help to regulate the immune response. Several subsets of T cells have been discovered, each subset having a different function. Some T cells produce chemical messengers that communicate with and attract other kinds of white cells whose assistance is needed. Some destroy virally infected cells and tumor cells. Some T cells, like B cells, are antigen-specific and destroy foreign antigens or the substances that carry them.
Almost all the activity of B cells and of some T cells targets specific antigens. That is, each time a new kind of foreign agent carrying a particular kind of foreign antigen invades the body, the immune system must produce a new round of B cells and T cells that attack only that antigen. It is estimated that a healthy immune system can create more than 100 million different types of antibodies. As some B cells and T cells mature, they begin to distinguish between the body's own cells and tissues that do not belong in the body. These immune cells become memory cells, because they remember a particular antigen so that the next time it appears, the immune response can mobilize quickly.
In some cases, people acquire permanent immunity * to a disease; for example, people who contract chickenpox usually will not have it again; or if they do, they have a much milder case.
When a B cell encounters a foreign invader, it starts to produce immunoglobulins or antibodies. An antibody is a very large protein molecule, and it has an antigen-binding site. Like a key designed to fit only a specific lock, an antibody locks onto a single type of antigen like an identifying marker. The combination of an antigen and an antibody (bound to each other) is known as an antigen-antibody complex or an immune complex. There is an additional part of the immune system called the complement system. Immune complexes attract complement proteins, which normally circulate in the bloodstream in an inactive form. The complement proteins are a group of proteins that work to complement or amplify the activities of antibodies.
Once the antibody binds to the antigen, a group of T cells known as helper T cells, which have become activated by the presence of the same antigen, secrete signaling molecules, small peptide molecules known as cytokines (SIGH-toh-kines). These signaling molecules stimulate other white blood cells (or they induce white blood cells to stimulate yet another set of white blood cells) in such a way that the activity of the antibodies is enhanced; or they may draw white blood cells to the site of infection or injury. Some T cells, called killer T cells, are specialized to attack and destroy target cells. At the same time, millions of antibodies circulate in the bloodstream to bind to other matching antigens they encounter and mount a larger attack.
Interferons are a group of cytokines that enhance and amplify the immune response; they are also used in medical therapies. The singular form of the word interferon is used to refer to these therapies. Interferon has been effective in slowing the growth of some cancers. Among the interferons are chemicals that draw phagocytes to the site of the infection. The complement system, a group of proteins that circulate in the blood in an inactive form, are activated by a large number of triggers, including the presence of antigen-antibody complexes at sites of infection. The proteins migrate to the site of an infection, where they intensify the already existing immune response and help to break up and dissolve microorganisms and foreign particles.
Other Defenses
In addition to physical barriers such as the skin and the mucous coating of inner membranes and the immune system, the body has several other defense mechanisms that fight harmful invading agents. Coughing and sneezing are automatic reflexes that can rid the body of irritants. Tears serve a similar purpose—they lubricate the eyes and wash away dust, pollen, smoke particles, and other irritants.
Immune Responses and Disease
Strong and healthy immune systems successfully ward off many diseases, particularly infections, but weakened immune systems are less able to do so. Thus a weakened immune system gives various diseases an opportunity to develop. Age affects the immune system's effectiveness and can be a cause of immune deficiency. Newborns and the elderly may have weak or impaired immune responses to infectious disease. The immune systems of newborns are not fully developed at birth but typically become stronger during the first year of life. Breastfeeding strengthens the infant's developing immune system.
A functioning immune system may be overactive and may mount an immune response against substances and tissues normally present in the body. In other words, the body attacks its own cells, giving rise to a number of immunological (IH-myoo-no-LAH-jihkul) disorders, which are called autoimmune (AW-toh-ih-MYOON) diseases. In autoimmune diseases, the body cannot distinguish between itself and foreign particles; it may turn its disease-fighting powers on its own tissues, blood, and organs.
As of the early 2000s, more than 80 chronic autoimmune diseases had been identified, affecting more than 23.5 million Americans. These diseases affect women more often than men. According to the American Autoimmune Related Diseases Association, in regard to the known chronic autoimmune diseases, about 75 percent of cases occur in women, and women appear to be most vulnerable to these diseases during their childbearing years when the levels of hormones are highest in their bodies.
Heredity seems to play a part as well. An individual may inherit a predisposition toward autoimmune diseases in general but may not have the same disease a close relative has. For example, a grandmother may have rheumatoid arthritis * , and her granddaughter may have lupus * . These diseases are related in many ways, but they are different diseases.
Some common medical conditions can put people at increased risk of infectious disease.
- People with diabetes * are especially prone to infection, in part because high blood sugar levels can interfere with white blood cell function.
- Individuals with chronic lung diseases are often at increased risk of pneumonia and bronchitis.
- Urinary tract infections tend to occur more frequently among people born with an abnormal urinary tract or among those who have experienced a urinary tract obstruction like a kidney stone
*
Compromised Immune Systems
Chronic diseases can wear down the immune system and make people more susceptible to infection. An immune system that is weakened in this way is said to be compromised. Following are major examples of compromised immune systems and the infectious diseases they can cause.
- Sickle cell anemia * causes damage to the spleen * . Because the spleen helps to protect the body against bacterial infections, sickle cell anemia leaves the body more vulnerable to them.
- Human immunodeficiency virus (HYOO-mun IH-myoo-no-dihFIH-shen-see) (HIV) infection badly damages and weakens the immune system. Persons with HIV are susceptible to life-threatening infections that take advantage of their failing immune system. These infections are called opportunistic infections because they occur only in persons whose weak immune systems give the disease organisms the opportunity to multiply and cause disease. In many cases, prompt diagnosis of such opportunistic infections and treatment of the underlying HIV with combinations of antiviral drugs have been shown to slow the immune system's deterioration.
- Certain drugs and drug therapy regimens can also undermine the work of the immune system. Anticancer drugs, which usually target rapidly dividing cells, often kill vital beneficial white blood cells in the bone marrow.
- Patients who receive organ transplants are given high dosages of corticosteroid * drugs, which suppress the immune response, in an attempt to prevent their bodies from rejecting the transplanted tissue, which typically is recognized by their bodies as foreign.
- Some people are born with immune system deficiencies, in which some component of the immune system is missing or does not function adequately. These persons are said to have primary immunodeficiencies. Most primary immunodeficiencies are genetic disorders. They are often diagnosed in children under the age of one year.
- Hypogammaglobulinemias (HI-po-GAH-muh-GLOH-byoo-lihNEE-me-uhs) are conditions in which the body has fewer antibodies than normal, a condition that always means an increased risk of infection. Hypogammaglobulinemias frequently result in a greater number of bacterial respiratory illnesses.
- Agammaglobulinemias (a-GAH-muh-GLOH-byoo-lih-NEE-meuhs) are conditions characterized by the absence of antibodies in the body and can cause severe and often fatal infections.
Other primary immune disorders include the following:
- Severe combined immune deficiency syndrome (SCID), in which an infant is born with a significant lack of both B cells and T cells, often leads to serious immunity problems; it occurs in 1 in 1 million births. During the first three months of life, babies with this condition can experience life-threatening infections and diseases, particularly sepsis * , pneumonia, and meningitis * . Common childhood diseases, such as chickenpox, can easily overwhelm these patients’ immune systems.
- Chronic granulomatous (GRAN-yoo-LO-muh-tus) disease occurs in males when the body's phagocytes are ineffective against certain bacteria and fungi. Patients develop recurrent and unusual skin, lymph node * , and other infections. Repeated infections can lead to granulomas (GRAN-yoo-LO-muhs), masses that develop in the skin, lungs, liver * , lymph nodes, and bones. They can be slow to heal and drain.
Other Influences on Body Defenses
Many environmental factors can affect the health of the immune system. In environments in which cigarette smoking is common, for example, people are at increased risk of lung cancer and respiratory ailments, both of which can lead to various secondary infections, including bronchitis. Secondhand smoke, or passive smoking, increases the incidence of respiratory infections in both infants and children. Children who are exposed to secondhand smoke may be predisposed to pneumonia, allergies, and asthma * , as well as repeated irritations of the eyes, nose, and mouth.
Nutrition, too, has an impact on the immune system. Diets deficient in a variety of nutrients, such as certain vitamins, minerals, or protein, can cause increased vulnerability to infection.
Medical Ways to Strengthen the Immune System
In some instances, a patient will be given antibodies produced by another person to boost his or her own immunity, which is known as passive immunity. Infants are born with immature immune systems, but receive important antibodies from their mothers prior to birth (across the mother's placenta * ) and after birth from breast milk. These maternal antibodies usually disappear from the infant's system within 6 to 12 months, but until that time they help to protect the infant against a range of infections, including pneumonia * , bronchitis * , influenza (more commonly known as the flu), and ear infections.
Doctors can also give gamma globulin (GAH-muh GLAH-byoo-lin) to patients; these are antibody preparations that offer temporary immunity to individuals who might need this protection. When individuals receive an immunization or are given a vaccine * , their body's immune system is being primed to recognize the particular bacterium or virus that forms the target of the vaccine. If these individuals are exposed to the same bacterium or virus at a later point, they will probably be able to fight off the infection again and not come down with the infectious disease.
See also AIDS and HIV Infection • Allergies • Asthma • Autoimmune Disorders: Overview • Bacterial Infections • Bronchiolitis and Infectious Bronchitis • Chickenpox (Varicella) • Diabetes • Ear Infections (Otitis) • Endocarditis, Infectious • Fever • Fungal Infections • Immune Deficiencies • Immunoglobulin Deficiency Syndromes • Infection • Lupus • Malnutrition • Meningitis • Parasitic Diseases: Overview • Pneumonia • Sepsis • Shingles (Herpes Zoster) • Staphylococcal Infections • Streptococcal Infections • Urinary Tract Infections • Vaccines and Immunization • Viral Infections