White blood cells, also called leukocytes, are much rarer than red blood cells, as there are only one or two of them for every thosand red blood cells in the human bloodstream. These cells are nearly colorless, are larger than red blood cells, condtain no hemoglobin, and have a nucleus. The chief function of white blood cells is to defend the body against invaders such as viruses, bacteria, and other foreign substances. Eosinophils Destroy larger parasites and modulate allergic inflammatory reactions. Basophils Release histamine and serotonin in certain immune reactions.
Lymphocytes Make immune responses.
All blood cells, including red bloods, develop from pluripotent hemopoietic stem cells, located in the liver for fetuses, and in the bone marrow for adults. B and T cells both play crucial roles in the immune response. T cells work on cell-mediated immune responses, while B cells help the immune system through humoral antibody responses. The main difference between B and T cells is how they are formed. Both bgin as stem cells in teh bone marrow, but future T cells migrate via the blood to the thymus.