Immunological reactions to blood and blood products.

All blood cells possess inherited antigens; their pattern is specific for any individual or groups of individuals, and remains constant throughout life. Although the majority of the antigens were first recognized in association with blood cells, it is now evident that many of them are also found in other tissues and in body fluids, and specific antigens have been demonstrated in plasma. Antibodies against blood cell or plasma antigens are usually formed when an antigen not present on the cells or in the plasma is introduced parenterally. There are, however, a few naturally occurring antibodies which are regularly found in human serum without any obvious previous exposure to the corresponding antigen. Most of these antibodies, whether naturally occurring or immune, are either IgM or IgG immunoglobulins.