Different immune cells have different functions and are designed to react to different situations. They communicate through chemical signals and work together to protect the body from disease-causing organisms and remove dead or damaged cells. A few of each type are always present in the body, but when foreign organisms invade, the appropriate cell types multiply quickly to take on the invaders.
Cell Types
The immune system contains a few different groups of cells, and there are many individual cell types within those groups. The major groupings are the phagocytes and lymphocytes. Phagocytes devour and destroy debris, bacteria, viruses, fungi, parasites and dead or dying cells in the body. Phagocytes can be divided into two main types: macrophages and dendritic cells. Granulocytes are related to phagocytes, but instead of engulfing invader, they release granules to destroy organisms they encounter. Lymphocytes are the defensive cells that react to foreign invaders by producing antibodies, calling other cells to the area and directly attacking the invasive organisms or particles. The two major types of lymphocyte are T cells and B cells.
Lymphocytes
The T and B cell lymphocytes are indistinguishable under a microscope, but carry out different functions. B cells are the antibody-producing cells of the body. These antibodies attach to foreign particles called antigens, which may be free floating or may be proteins on the surface of attacking organisms, such as bacteria. When antibodies bind to antigens, it signals the rest of the immune system to come destroy the invader. T cells, on the other hand, directly attack foreign organisms and particles. They also produce cytokines, which set off chemical cascades that stimulate other immune cells to respond to the infection.
Phagocytes and Granulocytes
The phagocytes called macrophages and dendritic cells are the largest types of immune cells. These cells physically engulf and digest invaders. They also send out an alert to the rest of the immune system in order to stimulate a complete immune response. The granulocytes eosinophils and basophils spray their contents onto invading organisms or damaged cells. Neutrophils ingest the invaders and use their granules to destroy the ingested microbes, making them both granulocytes and phagocytes. Mast cells live outside the bloodstream, and their granules induce inflammation upon release.
Other Immune Cells
Some other immune cells do not fit into the groupings that most immune cells do. Lymphokine-activated killers and natural killer cells are two such immune cells. Both of these types work against cancer cells in the body or cells that have been infected with certain viruses instead of working directly against foreign organisms. They may also secrete cytokines that attract other types of immune cells to help them destroy the damaged or destructive cells.
Interactions
All of the cells of the immune system work together to protect the integrity of the entire body. Often, the B cells and T cells are the first to encounter foreign organisms or antigens in the system and they signal the appropriate types of immune cells to converge in the area, depending on the specific nature of the invader. Immunization can prime the immune cells to recognize specific foreign substances, allowing the body to mount an immune response and destroy the invaders before they take hold and cause disease.
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