Immune System and Other Body Defenses: Overview

The skin and the immune system help protect the body from infection and disease.

The First Line of Defense: Physical Barriers

* , fungi * , parasites * , and viruses * , to nonliving toxins, chemicals, and drugs. Fortunately, the body has a number of safeguards located on its surface and internally that prevent most foreign invaders from entering or causing harm. The physical barriers that keep these invaders at bay are often referred to as the body's first line of defense. The skin is the largest organ in the body, and it provides both a physical and a chemical barrier against the outside world. The skin forms a protective layer that covers blood vessels, nerves, muscles, bones, and organs. Human skin serves also as a chemical shield; it produces peptide antibiotics * that it secretes in response to contact with some microorganisms.

However, no organism is sealed completely against its environment. The human body has direct contact with the external environment in multiple places, not just at the skin's surface. The walls of the lungs' airways and the mucus-coated surfaces of the intestines are barriers between the human body and potentially harmful agents from the external environment.

The moist linings of the lungs’ airways protectively secrete mucus (MYOO-kus), a sticky substance that traps microbes and other irritants that enter the body through the nose and throat. Tiny hairs called cilia (SIH-lee-uh) are threadlike projections that line the trachea and the lungs and propel foreign particles and mucus out of the respiratory tract and away from the lungs to a place where they can be swallowed safely. The majority of harmful microbes that reach the stomach are destroyed by stomach acids. In addition, tears and saliva both contain peptide antibiotics that destroy invaders.

Another important defense mechanism is the brain-blood barrier at the interface between circulating blood and the brain. The blood-brain barrier is a semipermeable layer of cells that lines individual capillaries in the brain. It is a specialized filter that surrounds the brain and spinal cord, and acts as a physical barrier to prevent proteins, toxins, and most microbes from entering brain cells while letting in glucose, the brain's most important nutrient.

The Second Line of Defense: The Immune System

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 *

The human body has several lines of defense against infection that work to prevent germs from invading the body, or to destroy the germs once they gain entrance.

The human body has several lines of defense against infection that work to prevent germs from invading the body, or to destroy the germs once they gain entrance.
Illustration by Electronic Illustrators Group. © 2016 Cengage Learning®.

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.

Targeting antigens with antibodies

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.

T cells and cytokines

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.

Autoimmune diseases

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.

Medical conditions that increase the risk of infectious disease

Some common medical conditions can put people at increased risk of infectious disease.


Books and Articles

Abbas, Abul K., et al. Basic Immunology: Functions and Disorders of the Immune System. 5th ed. St. Louis, MO: Elsevier, 2016.

Parham, Peter. The Immune System. 4th ed. New York: Garland Science, 2014.


American Autoimmune Related Diseases Association. “Autoimmune Disease in Women.” (accessed March 30, 2016).

MedlinePlus. “Autoimmune Disorders.” U.S. National Library of Medicine, National Institutes of Health. (accessed March 30, 2016).

United Network for Organ Sharing. “Transplantation.” (accessed March 30, 2016).


American Association of Immunologists. 1451 Rockville Pike, Suite 650, Rockville, MD 20852. Telephone: 301-634-7178. Website: (accessed March 30, 2016).

American Autoimmune Related Diseases Association. 22100 Gratiot Ave., Eastpointe, MI 48021. Telephone: 586-776-3900. Website: (accessed March 30, 2016).

Cancer Research UK. 407 St. John St., London, EC1V 4AD, UK. Telephone: 0300-123-1022. Website: (accessed March 30, 2016).

Eunice Kennedy Shriver National Institute of Child Health and Human Development. 31 Center Dr., Building 31, Room 2A32, MSC 2425, Bethesda, MD 20892-2425. Toll-free: 800-370-2943. Website: (accessed March 30, 2016).

Genetic Alliance. 4301 Connecticut Ave. NW, Suite 404, Washington, DC 20008. Telephone: 202-966-5557. Website: (accessed March 30, 2016).

* bacteria (bak-TEER-ee-a) are single-celled microorganisms, which typically reproduce by cell division. Some, but not all, types of bacteria can cause disease in humans. Many types can live in the body without causing harm.

* fungi (FUNG-eye) are microorganisms that can grow in or on the body, causing infections of internal organs or of the skin, hair, and nails.

* parasites (PAIR-uh-sites) are organisms such as protozoa (one-celled animals), worms, or insects that must live on or inside a human or other organism to survive. An animal or plant harboring a parasite is called its host. Parasites live at the expense of the host and may cause illness.

* viruses (VY-rus-sez) are tiny infectious agents that can cause infectious diseases. A virus can only reproduce within the cells it infects.

* antibiotics (an-tie-by-AH-tiks) are drugs that kill or slow the growth of bacteria.

* antigens (AN-tih-jens) are substances that are recognized as a threat by the body's immune system, which triggers the formation of specific antibodies against the substance.

* immune system (im-YOON SIStem) is the system of the body composed of specialized cells and the substances they produce that helps protect the body against disease-causing germs.

* phagocytes (FAG-oh-sites) are cells that protect the body by ingesting and destroying bacteria and harmful foreign particles.

* bone marrow is the soft tissue inside bones where blood cells are made.

* lymphatic system (lim-FAH-tik) is a system that contains lymph nodes and a network of channels that carry fluid and cells of the immune system through the body.

* histamine (HIS-tuh-meen) is a substance released by the body during inflammation. It causes blood vessels to expand and makes it easier for fluid and other substances to pass through vessel walls.

* immunity (ih-MYOON-uh-tee) is the condition of being protected against an infectious disease. Immunity often develops after a germ has entered the body.

* rheumatoid arthritis (ROOmah-toydar-THRY-tis) is a chronic disease characterized by painful swelling, stiffness, and deformity of the joints.

* lupus (LOO-pus) is a chronic or long-lasting disease that causes inflammation of connective tissue, the material that holds together the various structures of the body.

* diabetes (DYE-uh-BEE-teez) is a condition in which the body's pancreas does not produce enough insulin or the body cannot use the insulin it makes effectively, resulting in increased levels of sugar in the blood.

* kidney stone is a hard structure that forms in the urinary tract. This structure is composed of crystallized chemicals that have separated from the urine. It can obstruct the flow of urine and cause tissue damage and pain as the body attempts to pass the stone through the urinary tract and out of the body.

* sickle cell anemia, also called sickle cell disease, is a hereditary condition in which the red blood cells, which are usually round, take on an abnormal crescent shape and have a decreased ability to carry oxygen throughout the body.

* spleen is an organ in the upper left part of the abdomen that stores and filters blood. As part of the immune system, the spleen also plays a role in fighting infection.

* corticosteroid (KOR-ti-ko-STERoid) is one of several medications that are prescribed to reduce inflammation and sometimes to suppress the body's immune response.

* sepsis is a potentially serious spreading of infection, usually bacterial, through the bloodstream and body.

* meningitis (MEH-nin-JY-tis) is an inflammation of the meninges, the membranes that cover and protect the brain and the spinal cord. Meningitis is most often caused by infection with a virus or a bacterium.

* lymph nodes (LIMF) are small bean-shaped masses of tissue containing immune system cells that fight harmful microorganisms. Lymph nodes may swell during infections.

* liver is a large organ located beneath the ribs on the right side of the body. The liver performs numerous digestive and chemical functions essential for health.

* bronchitis (brong-KYE-tis) is a disease that involves inflammation of the larger airways in the respiratory tract; it can result from infection or other causes.

* pneumonia (nu-MO-nyah) is inflammation of the lungs.

* placenta (pluh-SEN-ta) is an organ that provides nutrients and oxygen to a developing baby; it is located within the womb during pregnancy.

* asthma (AZ-mah) is a condition in which the airways of the lungs repeatedly become narrowed and inflamed, causing breathing difficulty.

* vaccine (vak-SEEN) is a preparation of killed or weakened germs, or a part of a germ or product it produces, given to prevent or lessen the severity of disease if a person is exposed to the germ itself. Use of vaccines for this purpose is called immunization.

Disclaimer:   This information is not a tool for self-diagnosis or a substitute for professional care.

(MLA 8th Edition)