8 Types of White Blood Cells

White blood cells are the defenders of the body. Also called leukocytes, these blood components protect against infectious agents (bacteria and viruses), cancerous cells, and foreign matter. While some white blood cells respond to threats by engulfing and digesting them, others release enzyme-containing granules that destroy the cell membranes of invaders.

White blood cells develop from stem cells in bone marrow. They circulate in blood and lymph fluid and may also be found in body tissues. Leukocytes move from blood capillaries to tissues through a process of cell movement called diapedesis. This ability to migrate throughout the body via the circulatory system allows white blood cells to respond to threats at various locations in the body.

Macrophages

Macrophage and Bacteria — This is a colored scanning electron micrograph (SEM) of Mycobacterium tuberculosis bacteria (purple) infecting a macrophage. The white blood cell, when activated, will engulf the bacteria and destroy them as part of the body's immune response. Science Photo Library/Getty Images

Monocytes are the largest of the white blood cells. Macrophages are monocytes that are present in nearly all tissue. They digest cells and pathogens by engulfing them in a process called phagocytosis. Once ingested, lysosomes within the macrophages release hydrolytic enzymes that destroy the pathogen. Macrophages also release chemicals that attract other white blood cells to areas of infection.

Macrophages aid in adaptive immunity by presenting information about foreign antigens to immune cells called lymphocytes. Lymphocytes use this information to quickly mount a defense against these intruders should they infect the body in the future. Macrophages also perform many functions outside of immunity. They assist in sex cell development, steroid hormone production, resorption of bone tissue, and blood vessel network development.

Dendritic Cells

Like macrophages, dendritic cells are monocytes. Dendritic cells have projections that extend from the body of the cell that are similar in appearance to the dendrites of neurons. They are commonly found in tissues in areas that come in contact with the external environment, such as the skin, nose, lungs, and gastrointestinal tract.

Dendritic cells help identify pathogens by presenting information about these antigens to lymphocytes in lymph nodes and lymph organs. They also play an important role in the tolerance of self antigens by removing developing T lymphocytes in the thymus that would harm the body's own cells.

B Cells

B cell Lymphocyte — B cells are a type of white blood cell involved in immune response. They account for 10 percent of the body's lymphocytes. Steve Gschmeissner/Brand X Pictures/Getty Images

B cells are a class of white blood cell known as a lymphocyte. B cells produce specialized proteins called antibodies to counter pathogens. Antibodies help identify pathogens by binding to them and targeting them for destruction by other immune system cells. When an antigen is encountered by B cells that respond to the specific antigen, the B cells rapidly reproduce and develop into plasma cells and memory cells.

Plasma cells produce large quantities of antibodies that are released into circulation to mark any other of these antigens in the body. Once the threat has been identified and neutralized, antibody production is reduced. Memory B cells help protect against future infections from previously encountered germs by retaining information about a germ's molecular signature. This helps the immune system to quickly identify and respond to a previously encountered antigen and provides long-term immunity against specific pathogens.

T Cells

Cytotoxic T Cell — This cytotoxic T cell lymphocyte kills cells infected with viruses, or are otherwise damaged or dysfunctional, through release of cytotoxins perforin and granulysin, which cause lysis of the target cell. ScienceFoto.DE Oliver Anlauf/Oxford Scientific/Getty Images

Like B cells, T cells are also lymphocytes. T cells are produced in bone marrow and travel to the thymus where they mature. T cells actively destroy infected cells and signal other immune cells to participate in the immune response. T cell types include:

Cytotoxic T cells: actively destroy cells that have become infected
Helper T cells: assist in the production of antibodies by B cells and help activate cytotoxic T cells and macrophages
Regulatory T cells: suppress B and T cell responses to antigens so an immune response does not last longer than necessary
Natural Killer T (NKT) cells: distinguish infected or cancerous cells from normal body cells and attack cells that are not identified as body cells
Memory T cells: help to quickly identify previously encountered antigens for a more effective immune response

Reduced numbers of T cells in the body can seriously compromise the ability of the immune system to perform its defensive functions. This is the case with infections such as HIV. In addition, defective T cells may lead to the development of different types of cancer or autoimmune diseases.

Natural Killer Cells

Natural Killer Cell Granule — This electron micrograph image shows a lytic granule (yellow) within the actin network (blue) at the immune synapse of a natural killer cell. Gregory Rak and Jordan Orange, Children's Hospital of Philadelphia

Natural killer (NK) cells are lymphocytes that circulate in the blood in search of infected or diseased cells. Natural killer cells contain granules with chemicals inside. When NK cells come across a tumor cell or a cell that is infected with a virus, they surround and destroy the diseased cell by releasing the chemical-containing granules. These chemicals break down the cell membrane of the diseased cell initiating apoptosis and ultimately cause the cell to burst. Natural killer cells should not be confused with certain T cells known as natural Killer T (NKT) cells.

Neutrophils

Neutrophil Cell — This is a stylized image of a neutrophil, one of the white blood cells of the immune system. Science Picture Co/Getty Images

Neutrophils are white blood cells that are classified as granulocytes. They are phagocytic and have chemical-containing granules that destroy pathogens. Neutrophils possess a single nucleus that appears to have multiple lobes. These cells are the most abundant granulocyte in blood circulation. Neutrophils quickly reach sites of infection or injury and are adept at destroying bacteria.

Eosinophils

Eosinophil Cell — This is a stylized image of an eosinophil, one of the white blood cells of the immune system. Science Picture Co/Getty Images

Eosinophils are phagocytic white blood cells that become increasingly active during parasitic infections and allergic reactions. Eosinophils are granulocytes that contain large granules, which release chemicals that destroy pathogens. Eosinophils are often found in connective tissues of the stomach and intestines. The eosinophil nucleus is double-lobed and often appears U-shaped in blood smears.

Basophils

Basophil Cell — This is a stylized image of a basophil, one of the white blood cells of the immune system. Science Picture Co/Getty Images

Basophils are granulocytes (granule containing leukocytes) whose granules contain substances such as histamine and heparin. Heparin thins blood and inhibits blood clot formation. Histamine dilates blood vessels and increases blood flow, which helps the flow of white blood cells to infected areas. Basophils are responsible for the body's allergic response. These cells have a multi-lobed nucleus and are the least numerous of the white blood cells.