Below is an instruction from Human. Write a response.
    ### Instruction:
    What is (are) familial hemophagocytic lymphohistiocytosis ?
    ### Response:
    Familial hemophagocytic lymphohistiocytosis is a disorder in which the immune system produces too many activated immune cells (lymphocytes) called T cells, natural killer cells, B cells, and macrophages (histiocytes). Excessive amounts of immune system proteins called cytokines are also produced.
    