- Lymphocytes are a type of white blood cell and are crucial components of the immune system. - The formation of blood cells, including lymphocytes, is called hematopoiesis. - In adults, hematopoiesis primarily occurs in the red bone marrow, which is found in the spongy tissue within bones, particularly in long bones (at the ends), vertebrae, ribs, pelvis, and sternum. - Lymphocytes also mature or are stored in secondary lymphoid organs like the spleen, lymph nodes, and thymus (though thymus is involved in T cell maturation, not primary formation from stem cells).