Hairy cell leukaemia. Computer illustration of an abnormal white blood cell (B- lymphocyte) from a patient with hairy cell leukaemia. These cells show characteristic hair-like cytoplasmic projections and ruffles on their surfaces. Leukaemia is a blood cancer in which the blood-producing tissue in bone marrow produces excessive numbers of immature white blood cells, which impair the function of normal blood cells. The immune system is thus weakened. Hairy cell leukaemia is a rare form of leukaemia that mostly affects men. Most patients survive for five years or more after diagnosis.