Cells of the immune system
There are two broad sub-classes of lymphocytes -- B cells and T cells. They all originate in the bone marrow; however, T cells undergo a process of maturation in the thymus gland.

Mature lymphocytes are all small cells with a deeply basophilic nucleus and scanty cytoplasm. B cells give rise to plasma cells which secrete antibodies. As for T cells, some (CD4+) secrete lymphokines which act on other cells involved in the immune response. Others (CD8+, cytotoxic) incude lysis in infected cells.