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In addition to the major body organs, a complex set of cells are involved in immune system activity. These cells are produced throughout the body and have been classified into multiple layers.

Most cells involved in immune system activity are types of white blood cells (also called leukocytes). While there are many different varieties of white blood cells, all of them work together against pathogens and harmful substances. In addition, many types are also capable of moving and acting independently to defend the body against foreign invaders.

Lymphocytes are one class of white blood cells. There are several types of cells considered lymphocytes that respond to infection in different ways, including:

• T cells. This type of cell exists in two forms: killer T cells and helper T cells. Killer T cells detect and directly attack antigens. Helper T cells recognize the presence of foreign organisms and stimulate B cells into action.
  
  
• B cells. Once signaled by helper T cells, they reproduce by cloning into plasma cells. The plasma cells produce chemical defenses (antibodies) against specific antigens. The body produces millions of types of B cells because each type responds to only one type of antigen.
  
  
• Memory cells. Following an infection, some B and T cells become memory cells. These cells remain in the system in case the patient is infected with the same antigen again. These memory cells are one factor that helps the body develop immunity to some diseases.

Phagocytes are another class of white blood cells. These cells engulf and digest foreign particles and organisms. Phagocytes called monocytes travel through the bloodstream and remove foreign bodies. When monocytes move into tissues, they undergo changes and become macrophages. After they become macrophages, they can remove antigens from body tissue.

Macrophages also serve as antigen presenting cells (APCs). These cells are located throughout the body, checking tissues for foreign antigens. When an APC encounters an antigen, it breaks the antigen down. The APC then moves molecules from the antigen to its surface so other parts of the immune system will recognize the antigen.

Neutrophils are another type of phagocyte and are one of the most common white blood cells. They engulf pathogens and then inject them with granules of chemicals to kill the organism. Because they use these granules, they are also classified as granulocytes. Other granulocytes include basophils and eosinophils.

Mast cells are also involved in immune system activity but are not blood cells. They are found in many tissues (e.g., lungs, skin, tongue) and release chemicals called histamines in response to infections, including the presence of certain allergens (e.g., pollen, dander, mold). Many chemicals other than blood cells and mast cells also participate in immune system activity. These chemicals work to weaken or kill pathogens and neutralize potential toxins.

Antibodies are the major response cell of the immune system. They are proteins produced by the plasma cells created from cloned B cells. There are millions of types of B cells and each can produce antibodies to respond to one specific antigen. Antibodies work in many ways to disrupt infecting organisms.

Antibodies are also known as immunoglobulins (Ig) and are divided into five classes: IgA, IgD, IgE, IgG and IgM. Each group may perform different functions to respond to antigens or what is perceived to be an antigen. For example, IgE is involved in allergic reactions and signals the production of histamines from mast cells. It is also believed to be active in the immune system response to parasitic infections. Other antibodies have other functions, both known and unknown.

Chemicals such as hormones are messenger chemicals also used in immune system activity. They are often released by one tissue to initiate or end activity by another tissue. The hormones generated by the immune system are called lymphokines. Some of these hormones are used by the body to either suppress or encourage immune system activity.

The complement system of proteins floats freely in the blood. When they encounter an antigen, they work with antibodies to cause the cells of the pathogen to burst. They also may signal that a cell is infected and needs to be removed from the body.

Histamines and leukotrienes are chemicals released by granulocytes and mast cells. They may be injected into a pathogen to kill it. They also cause the blood vessels to dilate, or widen, so more agents of the immune system can reach the site of infection.