immune system- the network of cells, molecules, and organs that act together to defend the body against foreign invaders usch as bacteria, viruses, and fungi.

Immunity- disease resistance provided by the immune system.

Innate (Nonspecific) Immunity- the first line of defense against foreign invaders composing of the skin, tears, mucus, saliva, and the inflammation of tissues that takes place after injury or infection.

Adaptive (Specific) Immunity- the second type of immunity that responds only if invader is present. Each adaptive immune response (i.r.) is tailored to act on a specific type of invader and usually attacks only nonself substances. Adaptive i.r. uses a chemical "memory" to remember antigens that assists it in responding more effectively to subsequent exposures.

Antigens- structures on the surfaces of invading organisms that trigger immune system reactions.

Humoral Immune Response- a type of adaptive immunity during which invaders acting outside cells, including bacteria and toxins are resisted.

Cell Mediated Immune Response- a second type of adaptive immunity during which cells that are capable of destroying other cells become active. This destructive activity is limited to cells infected with or producing a specific antigen. Cell mediated i.r. attacks viruses and some cancers.

Pathogens- disease-causing agents such as viruses, bacteria, cancers, and toxins.

Macrophage- a type of white blood cell that performs phagocytosis. Some macrophages play a part in innate immunity by surrounding, ingesting, and destroying bacteria and other foreign invaders. In the adaptive immune system, macrophages attach to antigens and deliver them to be destroyed by other parts of the system.

Phagocytosis- literally means, "cell eating." the process by which certain macrophages ingest foreign invaders.

Lymphocyte- a type of white blood cell that identifies and destroys antigens. All lymphocytes begin as "stem cells" in bone marrow and either mature there or in the thymus. There are two types of lymphocytes, B and T cells.

B Lymphocyte- B lymphocytes, or B cells, are a subtype of white blood cells that mature in the bone marrow and make antibodies.

Antibodies- Y-shaped proteins, or immunoglobullins, that are produced by B cells. These proteins attach to antigens and destroy or help to destroy the foreign invaders.

T Lymphocyte- Both types of T lymphocytes mature in the thymus. The "T" stands for "thymus-derived."

thymus- a glandular organ located behind the breastbone