White blood cells, also known as leukocytes, function to protect the body from foreign invaders such as viruses, bacteria, parasites and fungi. White blood cells consist of five different types of cells including lymphocytes, monocytes, neutrophils, eosinophils and basophils. The immature and undifferentiated cells in the bone marrow, known as stem cells, continually produce new white blood cells at a rate of about 100 billion a day, according to the Merck Manual. A decrease in the number of white blood cells can be the result of malfunctioning stem cells or disorders that kill certain types of white blood cells.
Neutropenia
Neutrophils make up about 45 to 75 percent of the total white blood cell count, according to the Merck Manual. A diagnosis of neutropenia in an adult means that the blood contains fewer than 1700 neutrophils per microliter, and severe neutropenia is when the count is lower than 500 neutrophils per microliter, according to MayoClinic.com. Bacterial infections, allergic reactions, autoimmune diseases and certain medications can all lead to a drop in neutrophils, which causes a decrease in the total number of white blood cells. Because neutrophils protect the body from bacteria, patients with neutropenia become vulnerable to serious bacterial infections such as pneumonia.
Eosinophilic Disorders
Although eosinophils account for only about seven percent of the total number of white blood cells, according to the Merck Manual, conditions that decrease the number of eosinophils can affect overall immune responses. An increase in stress hormones such as cortisol, associated with a condition known as Cushing's syndrome, can cause a decrease in the number of eosinophils. Prolonged treatment with corticosteroids, medications that reduce inflammation, can also decrease the number of eosinophils.
Basophilic Disorders
Basophils, which account for less than three percent of the total number of white blood cells, function in wound repair and release histamine in response to allergic reactions, according to the Merck Manual. Thyrotoxicosis, a life-threatening condition in which the thyroid gland produces too much thyroid hormone, can abnormally decrease the number of basophils, therefore contributing to an overall decrease in white blood cells.
Lymphocytopenia
The total white blood cell count consists of 20 to 40 percent lymphocytes, according to the National Heart Lung and Blood Institute. Fewer than 1000 lymphocytes per microliter is the definition of lymphocytopenia--a condition characterized by an abnormally low number of lymphocytes. Infectious diseases such as HIV, tuberculosis and hepatitis can cause lymphocytopenia. Other causes include autoimmune diseases, cancers and treatments such as chemotherapy.
Bone Marrow Disorders
Because bone marrow cells continually produce new white blood cells, disorders of the bone marrow can lead to abnormally low white blood cell counts. Aplastic anemia is a condition in which the stem cells become damaged and destroyed, leaving fewer stem cells to produce new blood cells. Those with aplastic anemia suffer from low red blood cells as well as a low white blood cell count. Myelodysplastic syndrome describes a group of bone marrow disorders in which the bone marrow contains enough stem cells, but they produce immature or abnormal blood cells, resulting in a low count of functional cells.
