T cells and B cells

Lymphocytes exist as one of two functionally
and morphologically different types, T lymphocytes
and B lymphocytes. T lymphocytes undergo
differentiation in the thymus during
embryonic and fetal development, thus the designation
T lymphocyte or T cell. B lymphocytes
differentiate in the bone marrow in mammals
and in the bursa of Fabricius in birds (thus the
designation B cells). A series of further differentiating
steps take place in the lymph nodes (T
cells) and in the spleen (B cells).