Anticoagulant -- A substance that hinders the clotting of blood.

Antigen -- A substance, usually a protein or carbohydrate, that stimulates an immune response. Blood group antigens are located mainly on the membrane of red blood cells.

Blood group -- One of the classes (e.g., A, B, AB, or O) into which individuals or their blood can be separated on the basis of the presence or absence of specific antigens in the blood. (Also called blood type) The major classifications are the ABO and Rh groups, but many less common blood group antigens have been identified.

Electrolytes -- A substance that dissolves into ions in solution and thus becomes capable of conducting electric currents. Many biological processes depend on these ions, which include sodium, potassium, calcium, magnesium, and chloride.

Plasma -- the pale yellow fluid component of whole blood that consists of water and its dissolved constituents including proteins (e.g., albumin, fibrinogen, and globulins), electrolytes (e.g. sodium and chloride), sugars, lipids, metabolic waste products, amino acids, hormones, and vitamins.

Red blood cell -- any of the hemoglobin-containing cells that carry oxygen to the tissues and are responsible for the red color of vertebrate blood (also called erythrocytes, red corpuscles, red cells). Blood groups antigens are often located on red blood cells.

Rh factor -- Any one or more genetically determined antigens present in the red blood cells of most persons and higher animals and capable of inducing intense immune reactions. Rh antigens were first identified in 1939 in Rhesus monkeys. In basic blood typing, the ABO blood groups are also designated as positive or negative for the Rh antigen (e.g., "A positive" or "O negative" blood)

White blood cell -- Any of the blood cells that are colorless, lack hemoglobin, contain a nucleus, and include lymphocytes, monocytes, neutrophils, eosinophils, and basophils. (Also called leukocytes or white blood corpuscles)