Immune System

The immune system is the body's natural defense system. It consists of organs and highly specialized cells, all working together to clear infection from the body. To accomplish this, the immune system relies on natural barriers, such as skin and mucus, to help block infection. It also relies on white blood cells and antibodies. Antibodies (also called immunoglobulin, or Ig) are large, Y-shaped proteins used to identify and remove germs, such as bacteria and viruses.


White blood cells travel throughout the body and are made in the bone marrow. Bone marrow is found in the center shafts of certain long, flat bones of the body. There are many types of white blood cells with different roles in fighting infection. These cells are grouped into 3 categories: lymphocytes, granulocytes, and monocytes/macrophages.


The 2 major classes of lymphocytes are B cells and T cells.

  • B cells produce antibodies that circulate in the blood and lymph streams. These antibodies attach to foreign germs to mark them for destruction by other immune cells. The types of antibodies made by B cells are:

  • IgG, the main infection-fighting antibody in the bloodstream.

  • IgA, which helps protect the gut and respiratory tract.

  • IgM, made at the early signs of infection.

  • IgD, the purpose of which is unknown.

  • IgE, which helps fight against parasites, but can also be elevated with allergies and other conditions.

  • T cells are responsible for cellular immunity. T cells tell other white blood cells to prepare to fight. T cells provide support to other cells to help them fight and kill infected cells in the body. There are 2 main classes of T cells:

  • Helper T cells (CD4-positive T cells). They alert B cells to start making antibodies. They activate other T cells and macrophages. They influence which type of antibody is produced.

  • Cytotoxic T cells (CD8-positive T cells). They become killer cells that attack and destroy infected cells.


  • These cells are attracted to sites of inflammation, injury, or infection.

  • They release chemicals to kill germs or clean up wounds.

  • They are the "front-line" fighters against infection.

  • They have a short life span in the bloodstream. New ones are constantly being made.


Monocytes travel in the blood and mature to become macrophages that travel in the tissues. These cells are very important in alerting the immune system about an infection. Macrophages are scavengers whose job is to engulf or eat up infecting germs and even infected cells.


People who have problems with the number of white cells being made and how well they do their jobs can be more prone to certain germs. This can be the cause of a decreased ability to fight infection (immunodeficiency). Problems in the immune system range from mild to severe. Your caregiver can help you determine if your immune system is working properly.