White blood cells (leukocytes) are major players in the immune system and help defend against infection and disease. There are several different types of white blood cells with different functions. Based on their functions, WBC can be divided into two main groups; phagocytes and lymphocytes. Phagocytes are capable of engulfing/destroying waste products and foreign matter including infectious agents like bacteria. Lymphocytes guide and participate in the specific immune response. (Learn More about the immune system).
Phagocytes
Phagocytes are cells that are able to consume and break down foreign material, invading microbes and broken down cells/cell parts. They get their name from the the Greek 'phagein', to eat. Several different blood cells are able to perform this task.
Granulocytes (neutrophils, basophils, eosinophils)
These cells are called granulocytes because they contain numerous particles (granules). The granules contain chemicals and proteins that are released from the cells to help control inflammatory and immune functions. Granulocytes are also capable of engulfing and destroying foreign matter. They include the following types of cells :
**Normal values are noted in parenthesis (1)
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Neutrophils (3,000-7,000 cells/µL; 60-70% of WBC)
These cells make up more than half of the white blood cells in circulation. They are also called segs or polys, because of the unusual (segmented or polymorphic) structure of their nuclei. Neutrophils are usually the first cells to arrive at the scene of infection or inflammation (usually within 90 min). They engulf and destroy foreign matter and then die, forming pus. In cases of increased demand for neutrophils, immature neutrophils may be released from the bone marrow. These immature cells are commonly called bands or stabs because of their appearance.
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Eosinophils (50-400 cells/µL; 1-5% of WBC)
The role of eosinophils is not yet completely understood. They are known to play a role in parasitic infections and allergic reactions. People with chronic allergic reactions (such as asthma) usually have a higher number of circulating eosinophils.
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Basophils (25-100 cells/µL; 0-.75% of WBC)
These cells are stimulated by other immune cells and play a role in systemic allergic reactions.
Agranulocytes (monocytes, macrophages)
These cells are called agranulocytes because they lack the granules of granulocytes. Agranulocytes are capable of engulfing and destroying foreign matter.
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Monocytes/macrophages (100-800 cells/µL; 3-7% of WBC)
These cells are the largest type of white blood cell. They move through the bloodstream and eventually end up in the tissues. Once they leave the bloodstream and enter tissues they mature into macrophages. There are actually several different types of macrophages with different functions, each one specific to the type of tissue in which they are living (examples: macrophages in bone are called osteoclasts and macrophages in neural tissue are called microglia). After neutrophils, macrophages are usually the second cells on the scene of a problem. They engulf and process foreign matter so that lymphocytes (see next page) can recognize it and mount a specific defense.
**Normal values are noted in parenthesis
Lymphocytes (1,000-4,000 cells/µL; 25-33%)
These are recognition cells responsible for initiating the specific immune response of the immune system. Lymphocytes are the second most common (neutrophils are most common) circulating white blood cells. There are three major types of lymphocytes, B cells, T cells, and natural killer cells (NK cells). These cells are further divided into subtypes based on their function.
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T Cells (800-3,200 cells/µL; 80% of total lymphocytes)
Formed by the pluripotent stem cells in the bone marrow, T-cells mature in the thymus (a small organ located in the upper portion of the chest). There are two subtypes of T cells; cytotoxic T cells (CD8 cells) and helper T cells (CD4 cells).
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Cytotoxic T cells (CD8 cells) bind specifically to target cells; virally infected cells, cancer cells, or any foreign cells. After binding, cytotoxic T cells directly destroy the target cell.
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Helper T cells (CD4 cells) release special chemicals that help activate other cells of the immune response, including; B cells, cytotoxic T cells, other helper T cells, NK cells, and macrophages.
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B Cells (100-600 cells/µL; 10-15% of total lymphocytes). These cells are produced from the pluripotent stem cells in the bone marrow and stay in the marrow to mature. B cells are in charge of antibody production. B cells bind to their specific antigens, become activated 'plasma' cells and secrete large amounts antibodies.
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Natural Killer Cells (50-400 cells/µL; 5-10% of total lymphocytes). Natural killer cells (NK cells) bind to virally infected cells and cancer cells and directly kill them.
When it is useful, blood tests can break down the total WBC count and measure the amounts of the individual cell types as a percentage of the total white cell count. This is called a differential cell count, often called white blood cell count with differential.
Total White Blood Cell Count (4,500-11,000 cells/µL)
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