Blood carries immune system cells, known as white cells or leukocytes. Some examples of white cells are phagocytes and lymphocytes.
Phagocytes ingest and dissolve pathogens, as well as strange particles or dying cells.
Lymphocytes are divided into T cells, B cells, and natural killer cells (NK). B cells produce antibodies to respond to foreign cells, T cells attack pathogens using enzymes and other non-antibodies responses, while NK cells combat tumors and cells infected by viruses.