White blood cells, which function within the immune system to protect the body from foreign invaders, consist of five types of cells including lymphocytes, monocytes, basophils, neutrophils and eosinophils. The bone marrow continually produces new white blood cells to replace old, damaged or dying cells. Conditions and diseases that affect the bone marrow's ability to function or that kill white blood cells faster than the bone marrow produces new ones, can cause a low white blood cell count, defined by MayoClinic.com as less than 3,500 white blood cells per microliter of blood.
Diminished Bone Marrow Function
The bone marrow consists of stem cells---unspecialized cells that can develop into different types of cells in response to signals from the body. Aplastic anemia describes a dysfunction of the bone marrow characterized by a decrease in the number of stem cells. Myelodysplastic syndrome describes a condition in which the bone marrow contains enough stem cells, but they fail to mature and transform into white blood cells. Both of these disorders result in a low white blood cell count.
Abnormal Cell Growth
The stem cells in the bone marrow can produce an abnormal type of white blood cell, known as a leukemic cell. Because leukemic cells fail to die, they increase in numbers, eventually taking over the other types of cells in the blood, including white blood cells. As the number of normal white blood cells decrease, the patient becomes vulnerable to infections.
Metastasized Tumor Cells
Cancer, the abnormal growth of cells, can occur in any type of tissue in the body. Cells on a tumor can break away and travel through the bloodstream to affect other areas of the body. These cells, known as metastasized tumor cells, can reach the bone marrow and interfere with the stem cell's ability to differentiate into white blood cells. MayoClinic.com reports that breast cancer and prostate cancer commonly metastasize to the bone marrow.
Immune System Malfunction
The immune system can malfunction, causing white blood cells to attack normal healthy cells in the body, resulting in diseases classified as autoimmune diseases. Some autoimmune diseases destroy white blood cells or attack the cells in the bone marrow resulting in a low number of white blood cells. Lupus describes a chronic inflammatory disease that occurs when the body's immune cells attack the joints, skin, kidneys, blood cells, heart and lungs, according to MayoClinic.com. The decrease in white blood cells caused by lupus increases the risk for infections, leaving the patient vulnerable to serious, life-threatening diseases such as pneumonia. Other autoimmune disorders such as rheumatoid arthritis can also cause a low white blood cell count.
Treatments
Treatments for other conditions can cause unwanted side effects, including a low white blood cell count. Chemotherapy, a treatment for cancer, utilizes drugs that target and kill rapidly dividing cells. Because the stem cells in the bone marrow divide rapidly to produce new blood cells, chemotherapy destroys these cells, resulting in fewer white blood cells. Radiation therapy, another treatment used for cancer, delivers high energy waves to the body that not only destroy cancer cells, but also destroy bone marrow stem cells.
