The major components of the immune system include which of the following?

A. T lymphocytes.

B. B lymphocytes.

C. Cytokines.

D. Macrophages. 

DISCUSSION: The development of the lymphoid system begins with a pluripotent stem cell in the liver and bone marrow of the fetus. With maturation of the fetus toward term, the bone marrow becomes the primary site for lymphopoiesis. It produces the T lymphocytes, B lymphocytes, and macrophages that are critical to the immune response. These cells then produce cytokines or soluble growth factors, which amplify the immune response.