 There are four major types of lymphocytes, many sub-types, and hundreds or thousands of lymphocyte cell types that have been identified by scientists.All are generated by normal or abnormal lymphopoiesis except for certain artificial strains created in the laboratory by development from existing strains.Although lymphocytes are usually considered mature (as seen in blood tests) they are certainly not inert but can and do get around the body to anywhere there is a need; and when such need arises, new rounds of 'downstream' lymphopoiesis such as cell multiplication and differentiation may arise, coupled with intense mitotic and metabolic activity.