White blood cells function to protect the body from foreign invaders such as bacteria, viruses, parasites and fungi. For the immune system to function properly, the body needs a sufficient number of white blood cells. Normally, stem cells in the bone marrow produce approximately 100 billion new white blood cells per day, according to the Merck Manual. Five major types of white blood cells exist, including neutrophils, eosinophils, basophils, lymphocytes and monocytes. A low level of any of these cells affects the body and indicates a disorder.
Leukopenia
Leukopenia describes a condition of a low level of white blood cells. Normally, an adult's white blood cell count ranges from 4,500 to 10,000 white blood cells per microliter, according to Medline Plus. A count of less than 3,500 indicates leukopenia, according to the Mayo Clinic. A low white blood cell count increases the risk of contracting infections. Since white blood cells consist of different subtypes, each with its own function within the immune system, a low white cell count usually occurs due to a decrease in only one type.
Neutropenia
Neutropenia describes a condition of a low number of neutrophils--a type of white blood cell that accounts for approximately 45 to 75 percent of all white blood cells according to the Merck Manual. Neutropenia develops when neutrophils are destroyed faster than they are produced by the bone marrow. Acute neutropenia, a sudden onset, causes a fever and painful sores around the mouth or anus. If left untreated, neutropenia can result in bacterial pneumonia or other severe infections.
Lymphocytopenia
Different types of lymphocytes, including B lymphocytes, T lymphocytes and natural killer cells, work together to protect the body from infection. Lymphocytes make up about 20 to 40 percent of the total white blood cell count, according to the National Heart, Lung and Blood Institute. Although lymphocytopenia often fails to cause any specific signs and symptoms, unusual infections, recurrent infections or infections that fail to go away often indicate lymphocytopenia.
Monocytopenia
Although monocytes account for only 1 to 10 percent of white blood cells, their function in the immune system remains important. Monocytes mature into macrophages, the scavenger cells of the immune system. Monocytes and macrophages help other white blood cells by removing dead and damaged tissues. A low level of monocytes in the blood, often caused by the release of bacterial toxins, can cause the buildup of debris in the cells, leading to disorders such as Gaucher's disease and Niemann-Pick disease.
