White blood cells are components of the blood that protect the body from infectious agents. Also called leukocytes, white blood cells play an important role in the immune system by identifying, destroying, and eliminating pathogens, damaged cells, cancer cells, and foreign bodies from the body. White blood cells originate from bone marrow stem cells and circulate in the blood and lymphatic fluid. Leukocytes can leave blood vessels to migrate to body tissues. White blood cells are classified by the apparent presence or absence of granules (sacs that contain digestive enzymes or other chemicals) in their cytoplasm. A white blood cell is considered a granulocyte or an agranulocyte.


There are three types of granulocytes: neutrophils, eosinophils, and basophils. As seen under a microscope, the granules in these white blood cells are apparent when stained.

  • Neutrophils: These cells have a single nucleus that appears to have multiple lobes. Neutrophils are the most abundant granulocytes in the bloodstream. They are chemically attracted to bacteria and migrate through tissue to the site of infection. Neutrophils are phagocytic because they engulf the target cell (bacteria, diseased or dead cell, etc.) and destroy it. When released, the neutrophil granules act as lysosomes to digest cellular macromolecules. The neutrophil is also destroyed in the process.
  • Eosinophils: The nucleus of these cells is double-lobed and often appears U-shaped in blood smears. Eosinophils are often found in the connective tissues of the stomach and intestines. Eosinophils are phagocytic and primarily target antigen-antibody complexes. These complexes are formed when antibodies bind to antigens to identify them as substances to be destroyed. Eosinophils become increasingly active during parasitic infections and allergic reactions.
  • Basophils: Basophils are the least numerous of the white blood cells. They have a multilobed nucleus and their granules contain substances such as histamine and heparin. Heparin thins the blood and inhibits the formation of blood clots. Histamine dilates blood vessels, increases the permeability of capillaries, and increases blood flow, which helps transport leukocytes to infected areas. Basophils are responsible for the body’s allergic response.


There are two types of agranulocytes, also known as non-granular leukocytes: lymphocytes and monocytes. These white blood cells appear to have no obvious granules. Agranulocytes typically have a large nucleus due to the lack of notable cytoplasmic granules.

  • Lymphocytes: After neutrophils, lymphocytes are the most common type of white blood cell. These cells are spherical in shape with large nuclei and very little cytoplasm. There are three main types of lymphocytes: T cells, B cells, and natural killer cells. T and B cells are critical for specific immune responses. Natural killer cells provide nonspecific immunity.
  • Monocytes:These cells are the largest of the white blood cells. They have a single, large nucleus that can be shaped in a variety of ways. The nucleus often appears to be kidney-shaped. Monocytes migrate from the blood to the tissues and become macrophages and dendritic cells. Macrophages are large cells present in almost all tissues. They actively act phagocytic functions. Dendritic cells are commonly found in tissues located in areas that come into contact with antigens from the external environment. They are found on the skin, internally in the nose, lungs, and gastrointestinal tract. Dendritic cells function primarily to present antigenic information to lymphocytes in lymph nodes and lymphatic organs. This helps in the development of antigenic immunity.

White blood cell production

White blood cells are produced by the bone marrow within the bone. Some white blood cells mature in the lymph nodes, spleen, or thymus gland. The lifespan of mature leukocytes ranges from a few hours to several days. Blood cell production is often regulated by body structures such as the lymph nodes, spleen, liver, and kidneys. During times of infection or injury, more white blood cells are produced and present in the blood. A blood test known as a leukocyte or white blood cell count is used to measure the number of white blood cells in the blood. Typically, there are between 4,300-10,800 white blood cells present per microliter of blood. A low WBC count can be due to disease, radiation exposure, or bone marrow deficiency.

Other types of blood cells

  • Red blood cells: These biconcave-shaped cells carry oxygen to the cells and tissues of the body through the bloodstream. They also carry carbon dioxide to the lungs.
  • Platelets: These blood cells are vital for the clotting process, which is necessary to prevent blood loss.
Samantha Robson
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Dr. Samantha Robson ( CRN: 0510146-5) is a nutritionist and website content reviewer related to her area of ​​expertise. With a postgraduate degree in Nutrition from The University of Arizona, she is a specialist in Sports Nutrition from Oxford University and is also a member of the International Society of Sports Nutrition.

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