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  • Reviewed on 3/08
    By Dr. Alam

    • Rafeul Alam, MD

      Rafeul Alam, MD, PhD  
      Dept. of Medicine
      Chief, Division of Allergy & Clinical Immunology

      View full profile

About the Immune System

The immune system normally protects your health in two very important ways. First it recognizes "foreign invaders" like bacteria, viruses and other infectious organisms. Once the invaders are recognized, the immune system reacts by sending special cells and proteins to eliminate the infection. This is called the immune response.

The immune system is not located in just one part of the body. The thymus, liver, bone marrow, tonsils, lymph nodes, spleen and blood all have a role in the immune response. These organs help make, store and transport the cells and proteins that fight foreign invaders.

The major cells and proteins of the immune system are:

  • B-cells (also called B-lymphocytes)
  • T-cells (also called T-lymphocytes)
  • Phagocytes- "Eater Cells"
  • Complement-Special Proteins



B-cells make antibodies, also called immunoglobulins or gammaglobulins. Antibodies surround and coat the cells of foreign invaders. This weakens the invader cells. Some examples of antibodies are IgM, IgG, IgA, and IgE. IgM antibodies travel to the site of infection when it first begins. IgG antibodies are found in the blood and tissues. IgA antibodies are in secretions like tears, saliva, mucus, and gastric juices. Although not fully understood, IgE antibodies may play a role in fighting intestinal parasites. In atopic individuals (those who suffer from IgE-mediated allergies), IgE is the antibody that attaches to mast cells thereby priming them to release mediators such as histamine and leukotrienes. These mediators are released from mast cells when in the presence of an allergen like ragweed and are responsible for causing allergy symptoms.



Three kinds of T-cells travel in the blood to infected areas. Helper T-cells tell the immune system when more help is needed. Killer T-cells attack foreign invaders like infectious organisms. When the job is done, regulatory T-cells tell the immune system that its work is finished until next time.



Phagocytes are in the blood and tissues. They eat and kill foreign invaders. Phagocytes send chemical signals to other phagocytes, and help make complement. When antibodies and/or complement have coated the foreign invaders, it is easier for phagocytes to eat them.



Complement is made up of 18 proteins in the blood. The presence of foreign invaders and antibodies tells complement to start its work. Like antibodies, complement coats foreign invaders so that they are easier for phagocytes to eat and kill. Complement also sends signals to phagocytes when more are needed.

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