Leukocytosis refers to an increase in the total number of WBCs due to any cause. From a practical standpoint, leukocytosis is traditionally classified according to the component of white cells that contribute to an increase in the total number of WBCs. Therefore, leukocytosis may be caused by an increase in (1) neutrophil count (ie, neutrophilia), (2) lymphocyte count (ie, lymphocytosis), (3) monocyte count (ie, monocytosis), (4) eosinophilic granulocyte count (ie, eosinophilia), (5) basophilic granulocyte count (ie, basophilia), or (6) immature cells (eg, blasts). A combination of any of the above may be involved.
White blood cells or leukocytes are the cells of the immune system. They protect the body against foreign materials and infections. They are divided into two categories: granulocytes and agranulocytes. Granulocytes are of three types: neutrophils, eosinophils and basophils. Lymphocytes and monocytes are the two types of agranulocytes. The normal count of white blood cells is somewhere between 4,000 and 10,000/mm3. They have a short lifespan, ranging from a few days to a few weeks. These cells offer defensive properties to blood in order to fight against infections and the invading foreign bodies such as bacteria and viruses. If the number of leukocytes increases to more than the normal count, the condition is known as leukemia. If the white blood cell count is below normal, it is known as leukopenia. There may be a decrease in individual leukocyte percentage, e.g. neutropenia, i.e., a decrease in number of neutrophils. The reduction of all types of white blood cells