The immune system evolved to discriminate self from nonself. The two arms of immunity work closely together, with an innate immune system being most active early in an immune response and adaptive immunity becoming progressively dominant over time. The major effectors of innate immunity are complement, granulocytes, monocytes/macrophages, natural killer cells, mast cells, and basophils. The major effectors of adaptive immunity are B and T lymphocytes. B lymphocytes make antibodies; T lymphocytes function as helper, cytolytic, and regulatory (suppressor) cells. These cells are important in the normal immune response to infection and tumors, but also mediate transplant rejection and autoimmunity. Immunoglobulins (antibodies) on the B lymphocyte surface are receptors for a large variety of specific structural conformations.