Allergen
A substance that causes an allergic response when it enters the body.
Allergy
An abnormal and often harmful response of the immune system to foreign substances, or antigens.
Antibody
A complex protein produced by the body to combat bacteria, virus, or other antigens.
Antigen
Any substance or entity, usually a protein, that causes the body's immune system to produce antibodies.
B cell (B lymphocyte)
A small white blood cell, produced in the bone marrow, that develops into an antibody-producing plasmacell.
BCG (bacille Calmette-Guerin)
Tuberculosis vaccine, which is also used to stimulate the immune system in patients with certain types of cancer.
Immune deficiency
A condition in which some portion of the immune response is weak or absent.
Immune response
The reaction of the immune system to foreign substances in the body.
Immune system
The body's mechanisms for protecting itself against viruses, bacteria, and other foreign substances.
Immunity
Protection, in particular, from infection by the response of the immune system.
Leukocytes
White blood cells including lymphocytes, polymorphonuclear leukocytes, and monocytes.
Lymph nodes (Lymph glands)
The tissues that supply lymphocytes to the bloodstream and remove bacteria and other foreign substances from the fluid that contains the lymphocytes.
Lymphocyte
A type of white blood cell made in lymphatic tissue (lymph nodes, thymus, tonsils, spleen, bone marrow, and Peyer's patches) that is responsible for specific immunity.
Passive immunity
Protection resulting from the transfer of antibodies from another person, as from a mother to her unborn child.
Primary immune deficiency
A disorder that results from an inherited defect in the immune system, such as severe combined immunodeficiency disease.
Primary immune response
The response by the immune system the first time an antigen is encountered.
Tolerance
Nonresponsiveness to a particular antigen or group of antigens.