Below is an instruction from Human. Write a response.
    ### Instruction:
    What is (are) Hairy cell leukemia ?
    ### Response:
    Hairy cell leukemia is a rare, slow-growing cancer of the blood in which the bone marrow makes too many B cells (lymphocytes), a type of white blood cell that fights infection. The condition is named after these excess B cells which look 'hairy' under a microscope.
    