Types and Function of White Blood Cells (WBCs)

White blood cells (WBCs) are a part of the immune system that helps fight infection and defend the body against other foreign materials. Different types of white blood cells are involved in recognizing intruders, killing harmful bacteria, and creating antibodies to protect your body against future exposure to some bacteria and viruses.

There are several different types of white blood cells including:

• Neutrophils - Roughly half of white blood cells are neutrophils. Neutrophils are usually the first cells of the immune system to respond in response to an invader such as a bacteria or a virus. As first responders, they also send out signals alerting other cells in the immune system to respond to the scene. You may be familiar with the appearance of neutrophils as they are the primary cells present in pus. Once released from the bone marrow these cells live for only around eight hours, but around 100 billion of these cells are produced by your body every day.
• Eosinophils - Eosinophils also play an important role in fighting off bacteria and are very important in responding to infections with parasites (such as worms). They are perhaps best known, however, for their role in allergy symptoms, when they essentially go overboard in mounting an immune response against something (like pollen) which it mistakenly believes is an invader. These cells account for no more than 5 percent of the white blood cells in your bloodstream but are present in high concentrations in the digestive tract.
• Basophils - Basophils, also accounting for only around 1 percent of white blood cells, are important in mounting a non-specific immune response to pathogens. These cells are perhaps best known for their role in asthma. When stimulated these cells release histamine among other chemicals. The products can result in inflammation and bronchoconstriction in the airways.
• Lymphocytes (B lymphocytes and T lymphocytes) - Lymphocytes are also very important in the immune system, with T cells being responsible for directly killing many foreign invaders. B lymphocytes (B cells), in contrast to the other types of white blood cells, are responsible for humoral immunity (in contrast to the non-specific immunity of other white blood cells.) They produce the antibodies that "remember" an infection and stand ready in case your body should be exposed. B lymphocytes play the main role in the efficacy of the majority of the current vaccines but in some cases (i.e. tuberculosis and pertussis vaccines), T lymphocytes are also very important.
• Monocytes - Monocytes are the garbage trucks of the immune system. Around 5 to 12 percent of white blood cells in your bloodstream are monocytes, but their most important function is to migrate into tissues and clean up dead cells (among other functions.)

White blood cells begin in the bone marrow in a process called hematopoiesis. All blood cells, including white blood cells, red blood cells, and platelets, descend from a common hematopoietic stem cell, or "pluripotent" stem cell. These stem cells evolve (differentiate) in different stages. 

The HSC cell first separates into the lymphoid cell line, via a lymphoid stem or progenitor cell and into the myeloid cell line, via myeloid stem or progenitor cell. The lymphoid stem or progenitor cell gives rise to lymphocytes specifically B lymphocytes or "B cells" and T lymphocytes (T cells). The myeloid stem or progenitor cells give rise to myeloblasts, which further differentiate into macrophages, monocytes, neutrophils, basophils, and eosinophils and into the precursors of red cells and platelets.

A normal white blood cell count is usually between 4,000 and 10,000 cells/MCL.

Though you may think of infections, there are many causes of an elevated white blood cell count. These can be increased by overproduction, or rather by the body releasing white blood cells early from the bone marrow. Stress of any form can also result in this release of white blood cells. Some causes of an increased white blood cell count include:

• Infections
• Cancers such as leukemias, lymphomas, and myelomas in which a greater number of white blood cells are manufactured.
• Inflammation such as inflammatory bowel disease and autoimmune disorders
• Trauma ranging from fractures to emotional stress
• Pregnancy - In pregnancy, the number of white cells is "normally" elevated
• Asthma and Allergies - With allergies, you will often see an increase in the type of white blood cells known as eosinophils
• Exercise

Conditions which may result in a low white blood cell count include:

• Severe infections
• Bone marrow damage or disorders including aplastic anemia, bone marrow "takeover" by blood cancers or metastatic cancer, or drug or chemical-related damage to the bone marrow
• Autoimmune diseases such as lupus
• Splenic "sequestration" where white blood cells are accumulated in the spleen.

The symptoms of a low white blood count can be understood by knowing the function of white blood cells. Our white blood cells are our bodies defense against infections. Some of the cells are part of our innate immune system, meaning they know from birth to attack foreigners, and others are part of our humoral, or learned immune system, and manufacturer antibodies after "seeing" a germ in order to be prepared for another attack by that germ ahead of time. Symptoms of infection may include:

• Fever
• Cough
• Pain or frequency of urination
• Blood in the stools
• Diarrhea
• Redness, swelling, or warmth in a region of infection

One of the most common and dangerous side effects of chemotherapy is due to its effect on white blood cells, particularly the type of white blood cells known as neutrophils. Neutrophils are essentially the "first responders" of our immune system. A decrease in neutrophils during chemotherapy, known as chemotherapy-induced neutropenia, carries the risk of serious infection. Not only is it more difficult for the body to fight off infections relative to someone without neutropenia, but bacteria which are normally not terribly harmful can cause serious infections.

From infection to cancers, white blood cells are involved in many functions in the body. These cells may also become diseased themselves. A deficiency of one type of all white blood cells may occur with several immunodeficiency syndromes. A surplus of a type of these cells (due to malignancy) is present in disorders such as leukemias and lymphomas. 

Examples: After his chemotherapy treatment, John was told that his white blood cell count was low and that he should try to stay away from people that are sick for a few days to reduce his risk of infection.