Cells that are part of the immune system. Most develop from stem cells in the bone marrow and become different types of white blood cells (the microglia of the brain originate in the yolk sack during embryonic development). Immune cells are broadly classified into innate and adaptive immune cells. Innate immune cells include neutrophils, basophils, mast cells, monocytes and eosinophils, dendritic cells, and macrophages. Adaptive immune cells include B-cells and T-cells. T-Cells and Natural Killer T-cells mediate important dialogues between innate (rapid) and adaptive (slower) immune responses. B-cells and T-cells can form long- term immunological memory.