Antigens and antibodies are important components of blood. Antigens are carbohydrate or protein molecules found on the surface of red blood cells, white blood cells, and platelets, as well as on body tissue, body fluids, and surfaces of bacteria and viruses. They can trigger the immune system to produce antibodies directed against the donor’s blood group antigens if the recipient does not have that antigen. Antibodies, on the other hand, are proteins found in plasma that recognize foreign substances, such as germs, and alert the immune system, which destroys them. They are part of the circulating plasma proteins known as immunoglobulins, which are classified by molecular size and weight and by several other biochemical properties. Most blood group antibodies are found either on immunoglobulin G (IgG) or immunoglobulin M (IgM) molecules, but occasionally the immunoglobulin A (IgA) class may exhibit blood group specificity. There are five main subtypes of antibodies: IgG, IgM, IgA, IgD, and IgE. IgG and IgM are the most abundant types of antibodies in plasma. IgM antibodies are usually naturally occurring following exposure to environmental substances that have a similar structure to a red cell antigen, while IgG antibodies are formed following exposure to foreign red cell antigens through transfusion or during pregnancy.