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Immune response

Alternative names

Innate immunity; Humoral immunity; Cellular immunity; Immunity; Inflammatory response; Acquired (adaptive) immunity


The immune response is the way the body recognizes and defends itself against microorganisms, viruses, and substances recognized as foreign and potentially 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. Some 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.

One's innate immunity are the barriers that keep harmful materials from entering your body and form the first line of defense in the immune response. Some of these barriers are: the skin, stomach acid , mucous (traps microorganisms 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. Innate immunity also includes those things that make humans resistant to many of the diseases of animals.

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.

The inflammatory response (inflammation) is part of innate immunity. It 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, resulting in localized 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 where these white blood cells surround, engulf, and destroy foreign substances is called phagocytosis, and the cells are collectively referred to as phagocytes. Phagocytes eventually die. Pus is formed from a collection of dead tissue, dead bacteria, and live and dead phagocytes.

In comparison to innate immunity, acquired (adaptive) immunity develops when the body is exposed to various antigens and builds a defense that is specific to that antigen.

Lymphocytes, a special type of white blood cell, contain subgroups, B and T lymphocytes, that are key players in acquired immune responses. 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 distinctive 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, adaptive immunity accounts for an individual who has had chickenpox for being so-called 'immune' to getting chickenpox again.

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 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 complication of blood administration. 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.


  • localized redness
  • pain in the area
  • swelling of the affected area
  • warmth of the affected area
  • pus (sometimes)
Note: In many cases, no observable symptoms develop.

Additional symptoms may include:
  • fever
  • general discomfort, uneasiness, or ill feeling ( malaise )
  • muscle aches
  • agitation or confusion
During an infection, a CBC usually shows increased numbers of white blood cells. A blood differential count may reveal an elevated percentage of phagocytes, indicating 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).

Usually, 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.

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 /hypersensitivity
  • anaphylaxis
  • autoimmune disorders
  • blood transfusion reaction
  • immunodeficiency disorders
  • serum sickness
  • transplant rejection
  • graft versus host disease

Update Date: 10/30/2003

Donald Accetta, MD, MPH. President, Allergy & Asthma Care, PC, Taunton, MA. Review provided by VeriMed Healthcare Network.

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Last updated: Tue, 06 Jan 2009 00:20:03 GMT