Motile to enter CT and epithelial tissue, but is not phagocytic. The large granular lymphocyte is the natural killer cell . Unlike granular leucocytes, small lymphocytes can be stimulated to enlarge and divide by antigens, cytokines, and some plant lectins. The complex role of lymphocytes in immune defences is outlined in Chapter 19 and Fig. Lymphocytes circulate in blood and lymph systems and migrate to CT and mucous membranes.
Some lymphocytes have a lifespan of months or years.
They amount to 25-35 per cent of the leucocytes. Cytoplasm is abundant, with a few granules that are precursors of many larger lysosomes seen in EM when the cell is actively phagocytic . Macrophages, by releasing cytokines after activation, coordinate inflammatory and defensive reactions. They comprise 3-10 per cent of the leucocytes. Consist of cytoplasm, organelles and inclusions, bounded by a cell membrane, reflecting their formation as pseudopodia breaking away from extravascular cells - megakaryocytes .