White Blood Cells—Granulocytes and Agranulocytes

White Blood Cells
This photomicrograph of a blood smear reveals the presence of a few white blood cells.

Dr. Candler Ballard / CDC

White blood cells are blood components 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 removing pathogens, damaged cells, cancer cells, and foreign matter from the body.

Leukocytes originate from bone marrow stem cells and circulate in blood and lymph fluid. Leukocytes are able to leave blood vessels to migrate to body tissues.

White blood cells are categorized by the apparent presence or absence of granules (sacs containing digestive enzymes or other chemical substances) in their cytoplasm. If they have granules, they are considered granulocytes. If they do not, they are agranulocytes.

Key Takeaways

  • The primary purpose of white blood cells is to protect the body from infection.
  • White blood cells are produced by bone marrow and their levels of production are regulated by organs such as the spleen, liver, and kidneys.
  • Granulocytes and agranulocytes are the two types of white blood cells or leukocytes.
  • Granulocytes contain granules or sacs in their cytoplasm and agranulocytes do not. Each type of granulocyte and agranulocyte plays a slightly different role in fighting infection and disease.
  • The three types of granulocytes are neutrophils, eosinophils, and basophils.
  • The two types of agranulocytes are lymphocytes and monocytes.

White Blood Cell Production

White blood cells are produced within bones by bone marrow and some then mature in the lymph nodes, spleen, or thymus gland. Blood cell production is often regulated by body structures such as the lymph nodes, spleen, liver, and kidneys. The life span of mature leukocytes can be anywhere from a few hours to several days.

During times of infection or injury, more white blood cells are produced and sent into the blood. A blood test known as a white blood cell count or WBC is used to measure the number of white blood cells present in the blood. There are between 4,300-10,800 white blood cells present per microliter of blood in the average healthy person.

A low WBC count may be due to disease, radiation exposure, or bone marrow deficiency. A high WBC count may indicate the presence of an infectious or inflammatory disease, anemia, leukemia, stress, or tissue damage.

Granulocytes

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 with multiple lobes. Neutrophils are the most abundant white blood cell in circulation. They are chemically drawn to bacteria and migrate through tissue toward infection sites. Neutrophils are phagocytic, meaning that they engulf and destroy target cells. When released, their granules act as lysosomes to digest cellular macromolecules, destroying the neutrophil in the process.
  • Eosinophils: The nucleus of these cells is double-lobed and appears U-shaped in blood smears. Eosinophils are usually found in connective tissues of the stomach and intestines. These are also phagocytic and primarily target antigen-antibody complexes formed when antibodies bind to antigens to signal that they should be destroyed. Eosinophils are most active during parasitic infections and allergic reactions.
  • Basophils: Basophils are the least numerous type of white blood cells. They have a multi-lobed nucleus and their granules contain immune-boosting compounds such as histamine and heparin. Basophils are responsible for the body's allergic response. Heparin thins the blood and inhibits blood clot formation while histamine dilates blood vessels to increase blood flow and the permeability of capillaries so that leukocytes may be transported to infected areas.

Agranulocytes

Lymphocytes and monocytes are the two types of agranulocytes or nongranular leukocytes. These white blood cells have no obvious granules. Agranulocytes typically have a larger nucleus due to the lack of noticeable 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 cellsB cells, and natural killer cells. T cells and B cells are critical for specific immune responses and natural killer cells provide nonspecific immunity.
  • Monocytes: These cells are the greatest in size of the white blood cells. They have a large, single nucleus that comes in a variety of shapes but is most often kidney-shaped. Monocytes migrate from blood to tissue and develop into either macrophages and dendritic cells. 
    • Macrophages are large cells present in nearly all tissues. They actively perform phagocytic functions. 
    • Dendritic cells reside most often in the tissue of areas that come into contact with external antigens. They are found in the skinlungs, gastrointestinal tract, and inner layers of the nose. Dendritic cells function primarily to present antigenic information to lymphocytes in lymph nodes and lymph organs to aid in the development of antigen immunity. Dendritic cells are so named because they have projections that are similar in appearance to the dendrites of neurons.