There are three different types of cells in the blood, the red blood cells, white blood cells, and the platelets. All these cells are produced in the bone marrow. It is the white blood cells or the leukocytes that are involved in the defense systems of the body. There are five types of leukocytes. These are grouped into two distinct classes based on their morphology. The "polymorphonuclear granulocytes" have nuclei and specific granules inside the cells. The neutrophils, eosinophils, and basophils comprise this class of leukocytes. The cells that lack nuclei and have no cell-specific granules fall into the second category of leukocytes, known as "mononuclear agranulocytes". The monocytes and lymphocytes belong to this class.

The lymphocytes are the most abundant of the white blood cells. They are also the main functional cells of the immune system. There are two main types of lymphocytes, the T-lymphocytes, and the...