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White blood cells are much less numerous than red blood cells; the ratio between the two is around 1:700.  It is clear that white blood cells are much different than other cells of our body. These cells have an important role, so a doctor will often wish to see a patient’s white blood cell count. But what exactly is a low white blood cell count, since this is a common test result?
What Are White Blood Cells?
There are several different kinds of white blood cells, each of them specializing in defending the human organism from a particular infection. It’s like having an army, navy, marines and air force in reserve for each type of invasion. Depending on the type of infection, the navy or air force will activate.
The white cells go wrong in leukemia.  Therefore, a simple blood count will eliminate the worry about leukemia if your doctor suspects it.
Different types of white blood cells have different roles. 
Lymphocytes are one group; there are several kinds of lymphocytes, although they all look alike under the microscope.
Each of these lymphocytes has a different function to perform.
The most common types of lymphocytes are B-lymphocytes, responsible for making antibodies.
T-lymphocytes or T-cells are also important, with several subsets of these cells.  Inflammatory T-cells recruit macrophages and neutrophils to the site of infection or other tissue damage. Cytotoxic T-lymphocytes kill virus-infected and potential tumor cells, while helper T-cells enhance the production of antibodies by B-cells. Although bone marrow is the ultimate source of lymphocytes, the lymphocytes that will become T-cells migrate from the bone marrow to the thymus, where they mature. Both B-cells and T-cells also take up residence in the lymph nodes, the spleen, and other tissues where they encounter antigens, continue to divide by mitosis, and mature into fully functional cells.
Monocytes leave the blood and become macrophages.  A single macrophage is commonly surrounded by several lymphocytes. Macrophages are large, phagocytic cells that engulf foreign material called antigens that enter the body, and also dead and dying cells of the body. Monocytes and their macrophage and dendritic-cell progeny serve three main functions in the immune system:
- Antigen presentation
- Cytokine production
Neutrophils are the most abundant (40% to 75%) of the white blood cells. They squeeze through the capillary walls and into infected tissue where they kill invaders such as bacteria and then engulf the remnants by phagocytosis. This is a never-ending task; even in healthy people's throats, nasal passages and the colon harbor vast numbers of bacteria. Most of these are commensals, and do us no harm — but that is because neutrophils keep them in check. However, heavy doses of radiation, chemotherapy, and many other forms of stress can reduce the numbers of neutrophils.  If that happens, formerly harmless bacteria begin to proliferate, and the resulting opportunistic infections can be life threatening.
Eosinophils' number in the blood is normally quite low, 0–450/µl. However, their numbers increase sharply in the case of certain diseases, especially infections by parasitic worms.  Eosinophils are cytotoxic, releasing the contents of their granules on the invader killing it.
Basophils' number also increases during infection.  Basophils leave the blood and accumulate at the site of the infection or other inflammation, discharging the contents of their granules, releasing a variety of mediators such as histamine, serotonin, prostaglandins, and leukotrienes. These substances increase the blood flow to the area and in other ways add to the inflammatory process. The mediators released by basophils also play an important part in some allergic responses such as hay fever and an anaphylactic response to insect stings.