Lifespan's A - Z Health Information Library

The immune response is how your body recognizes and defends itself against bacteria, viruses, and substances that appear foreign and harmful to the body.

The immune system protects the body from potentially harmful substances by recognizing and responding to so-called antigens. Antigens are large molecules (usually proteins) on the surface of cells, viruses, fungi, or bacteria. Non-living substances such as toxins, chemicals, drugs, and foreign particles (such as a splinter) can be antigens. Substances that contain these antigens are recognized and destroyed by the immune system. Even your own body cells have proteins that are antigens. These include a group of antigens called HLA antigens. Your immune system learns to see these antigens as normal and does not usually react against them.

BARRIERS AND INNATE IMMUNITY

Your immune system includes barriers that keep harmful materials from entering your body. These barriers -- part of your innate (with you from birth) immunity -- form the first line of defense in the immune response. Some of these barriers are the skin, stomach acid, mucus (which traps bacteria and small particles), the cough reflex, and enzymes in tears and skin oils. If an antigen gets past the external barriers, it is attacked and destroyed by other parts of the immune system.

BLOOD COMPONENTS

The immune system includes certain types of white blood cells. It also includes chemicals and proteins in the blood, such as complement proteins and interferon. Some of these directly attack foreign substances in the body, and others work together to help the immune system cells.

INFLAMMATION

The inflammatory response (inflammation) occurs when tissues are injured by bacteria, trauma, toxins, heat, or any other cause. Chemicals including histamine, bradykinin, serotonin, and others are released by damaged tissue. These chemicals cause blood vessels to leak fluid into the tissues, causing swelling. This helps isolate the foreign substance from further contact with body tissues.

The chemicals also attract white blood cells that "eat" microorganisms and dead or damaged cells. The process in which these white blood cells surround, engulf, and destroy foreign substances is called phagocytosis, and the cells are called phagocytes. Phagocytes eventually die. Pus is formed from a collection of dead tissue, dead bacteria, and live and dead phagocytes.

ACQUIRED IMMUNITY

Acquired immunity is when the body is exposed to various antigens and builds a defense that is specific to that antigen.

Lymphocytes are a special type of white blood cell. B lymphocytes (also called B cells) produce antibodies. Antibodies attach to a specific antigen and make it easier for the phagocytes to destroy the antigen. T lymphocytes (T cells) attack antigens directly, and provide control of the immune response. B cells and T cells develop that are specific for ONE antigen type. When you are exposed to a different antigen, different B cells and T cells are formed.

As lymphocytes develop, they normally learn to recognize the body's own tissues (self) as different from tissues and particles not normally found in your body (non-self). Once B cells and T cells are formed, a few of those cells will multiply and provide "memory" for the immune system. This allows the immune system to respond faster and more efficiently the next time you are exposed to the same antigen, and in many cases will prevent you from getting sick.

For example, an individual who has had chickenpox is immune to getting chickenpox again.

PASSIVE IMMUNITY

Passive immunity involves antibodies that are produced in someone's body other than your own. Infants have passive immunity because they are born with antibodies that are transferred through the placenta from the mother. These antibodies disappear between 6 and 12 months of age. Gamma globulin is another form of getting passive immunity that is given by a doctor. Its protection is also temporary.

IMMUNE SYSTEM DISORDERS AND ALLERGIES

Immune system disorders occur when the immune response is inappropriate, excessive, or lacking. Allergies involve an immune response to a substance that, in the majority of people, the body perceives as harmless. Transplant rejection involves the destruction of transplanted tissues or organs and is a major complication of organ transplantation. Blood transfusion reaction is a problem caused by receiving blood. Autoimmune disorders (such as systemic lupus erythematosus and rheumatoid arthritis) occur when the immune system acts to destroy normal body tissues. Immunodeficiency disorders (such as inherited immunodeficiency and AIDS) occur when there is a failure in all or part of the immune system.

SIGNS OF INFLAMMATION:

• Redness in the area
• Pain in the area
• Swelling of the affected area
• Warmth of the affected area
• Pus (sometimes)

In many cases, however, there are no noticeable symptoms.

Additional symptoms may include:

• Fever
• General discomfort, uneasiness, or ill feeling (malaise)
• Muscle aches
• Agitation or confusion

TESTS

During an infection, a complete blood count (CBC) usually shows increased numbers of white blood cells. A blood differential count may reveal a high percentage of phagocytes (white blood cells that eat other cells), meaning that the body is responding to a need to fight infection.

If a problem is suspected, other tests may be performed to determine complement levels and the levels of specific immunoglobulins (antibodies).

THERAPIES

Usually, the action of the immune response is desired. In some cases, suppression of the immune system is necessary -- for example, in the treatment of autoimmune disorders or allergies. This is usually accomplished by administering corticosteroids or other immunosuppressive medications.

Suppression of the immune system may be an undesired side-effect of certain treatments or disorders.

Vaccination (immunization) is a way to trigger the immune response. Small doses of an antigen, such as dead or weakened live viruses, are given to activate immune system "memory" (activated B lymphocytes and sensitized T lymphocytes). Memory allows your body to react quickly and efficiently to future exposures. As noted above, this means that if you are exposed to a microorganism, it will be destroyed before it can cause illness.

Passive immunization involves transfusion of antiserum, which contains antibodies that are formed by another person or animal. It provides immediate protection against an antigen, but does not provide long-lasting protection. Gamma globulin and equine (horse) tetanus antitoxin are examples of passive immunization.

COMPLICATIONS

An efficient immune response protects against many diseases and disorders. Inefficient immune response allows diseases to develop. Inadequate, inappropriate, or excessive immune response causes immune system disorders.

Complications related to altered immune response include:

• Disease development
• Allergy or hypersensitivity
• Anaphylaxis
• Autoimmune disorders
• Blood transfusion reaction
• Immunodeficiency disorders
• Serum sickness
• Transplant rejection
• Graft versus host disease