ABSTRACT
In higher forms of life, elaborate mechanisms have evolved permitting defense against foreign substances in many subtle ways. The major responsibility for the execution of this form of adaptive response goes to the lymphoid system. All cells of the lymphoid system have their origin in reticulum cells which are predominantly found in the bone marrow, though they exist in other lymphoid tissues. Stem cells originate in the embryonic yolk sac and migrate to hematopoietic colonies in fetal liver and bone marrow, then onto other lymphoid tissues where further differentiation occurs. The commitment to lymphoid or myeloid differentiation apparently is regulated by inductive factors present in the microenvironment of these organs. Lymphocytes are seeded from the thymus to peripheral lymphoid tissues and these are referred to as "T" lymphocytes. Antigen-binding lymphocytes are found in the blood of normal adults but not in the blood of patients with agammaglobulinemia.
