White blood cell. Coloured scanning electron micrograph (SEM) of a lymphocyte cell, a type of white blood cell and part of the immune system. Lymphocytes are divided into either Bcells or T cells. Bcells mature in bone marrow and are responsible for humoural immunity; they operate by recognising a specific site on the surface of a pathogen or foreign object (antigen), which they then bind to before producing antibodies to destroy that antigen. Tcells mature in the thymus and are involved in cellmediated immunity, which does not rely on antibodies to fight antigens, but rather the activation of macrophages, NKcells, cytotoxic Tlymphocytes and various cytokines.