The types of leukemia are grouped based on the type of white blood cell that is affected. Leukemia can start in lymphoid cells or myeloid cells. Leukemia that affects lymphoid cells is called lymphoid, lymphocytic or lymphoblastic leukemia. Leukemia that affects myeloid cells is called myeloid, myelogenous or myeloblastic leukemia.
Hairy cell leukemia is a rare type of leukemia. Abnormal B-lymphocytes (a type of white blood cell) are present in the bone marrow, spleen and peripheral blood. When viewed under a microscope, these cells appear to be covered with tiny hair-like projections.