Introduction to White Blood Cells
- The name 'white blood cell' comes from the appearance of a blood sample after centrifugation.
- White cells are found in the buffy coat, a thin layer of nucleated cells between red blood cells and blood plasma.
- The term 'leukocyte' directly reflects its description, derived from the Greek roots 'leuk-' meaning white and 'cyt-' meaning cell.
- The buffy coat may appear green if there are large amounts of neutrophils in the sample.
- Neutrophils produce the heme-containing enzyme myeloperoxidase, which gives a green color.
- All white blood cells are nucleated, distinguishing them from red blood cells and platelets.
- Types of leukocytes can be classified by structure (granulocytes or agranulocytes) or by cell lineage (myeloid cells or lymphoid cells).
- The five main types of white blood cells are neutrophils, eosinophils, basophils, lymphocytes, and monocytes.
- Monocytes and neutrophils are phagocytic, meaning they can engulf and destroy pathogens.
- Granulocytes have lobed nuclei and visible cytoplasmic granules, while agranulocytes have round nuclei and no visible granules.
Neutrophils
- Neutrophils are the most abundant white blood cell, constituting 60-70% of circulating leukocytes.
- They are first responders to microbial infection and defend against bacterial or fungal infections.
- Neutrophils have a multi-lobed nucleus, giving them the appearance of having multiple nuclei.
- Their cytoplasm may look transparent due to pale lilac granules.
- Neutrophils are active in phagocytosing bacteria and are present in large amounts in pus.
Lymphocytes
- Lymphocytes make up 30% of white blood cells and are divided into T cells, B cells, and natural killer cells.
- B cells release antibodies and assist in the activation of T cells.
- T cells include helper T cells, cytotoxic T cells, gamma delta T cells, regulatory T cells, and natural killer cells.
- Lymphocytes have deeply staining nuclei and play a role in both innate and adaptive immune responses.
- Memory cells can last for years, while other lymphocytes have a lifespan of weeks.
Monocytes
- Monocytes constitute 5.3% of white blood cells and differentiate into tissue resident macrophages.
- They migrate from the bloodstream to other tissues and play a role in immune defense.
- Monocytes have a kidney-shaped nucleus.
- They differentiate into tissue resident macrophages, such as Kupffer cells in the liver.
- The lifespan of monocytes ranges from hours to days.
Other White Blood Cells
- Eosinophils compose about 2-4% of white blood cells in circulating blood.
- Eosinophils primarily deal with parasitic infections and are the predominant inflammatory cells in allergic reactions.
- Basophils are responsible for allergic and antigen response by releasing histamine.
- Fixed macrophages in the liver, known as Kupffer cells, are an example of tissue-resident white blood cells.
- Symptoms of neutropenia are associated with the underlying cause of the decrease in neutrophils.
- Causes of lymphocytopenia include inherited immune deficiency, blood cell dysfunction, infectious diseases, medications, radiation, major surgery, and immune dysfunction.
- An increase in the number of white blood cells in circulation is called leukocytosis.
- Neutrophilia is an increase in the absolute neutrophil count in the peripheral circulation and is often secondary to inflammation.
- Eosinophilia is never a normal lab finding and requires investigation into the underlying cause.
- The complete blood cell count includes the overall white blood cell count and differential count, which counts each type of white blood cell.
- Reference ranges for blood tests specify the typical counts in healthy individuals.
White blood cells, also called leukocytes or immune cells also called immunocytes, are cells of the immune system that are involved in protecting the body against both infectious disease and foreign invaders. White blood cells include three main subtypes; granulocytes, lymphocytes and monocytes.
| White blood cell | |
|---|---|
 A scanning electron microscope image of normal circulating human blood. In addition to the irregularly shaped leukocytes, both red blood cells and many small disc-shaped platelets are visible. | |
| Details | |
| System | Immune system |
| Identifiers | |
| Acronym(s) | WBC |
| MeSH | D007962 |
| TH | H2.00.04.1.02001 |
| FMA | 62852 |
| Anatomical terms of microanatomy | |
All white blood cells are produced and derived from multipotent cells in the bone marrow known as hematopoietic stem cells. Leukocytes are found throughout the body, including the blood and lymphatic system. All white blood cells have nuclei, which distinguishes them from the other blood cells, the anucleated red blood cells (RBCs) and platelets. The different white blood cells are usually classified by cell lineage (myeloid cells or lymphoid cells). White blood cells are part of the body's immune system. They help the body fight infection and other diseases. Types of white blood cells are granulocytes (neutrophils, eosinophils, and basophils), and agranulocytes (monocytes, and lymphocytes (T cells and B cells)). Myeloid cells (myelocytes) include neutrophils, eosinophils, mast cells, basophils, and monocytes. Monocytes are further subdivided into dendritic cells and macrophages. Monocytes, macrophages, and neutrophils are phagocytic. Lymphoid cells (lymphocytes) include T cells (subdivided into helper T cells, memory T cells, cytotoxic T cells), B cells (subdivided into plasma cells and memory B cells), and natural killer cells. Historically, white blood cells were classified by their physical characteristics (granulocytes and agranulocytes), but this classification system is less frequently used now. Produced in the bone marrow, white blood cells defend the body against infections and disease. An excess of white blood cells is usually due to infection or inflammation. Less commonly, a high white blood cell count could indicate certain blood cancers or bone marrow disorders.
The number of leukocytes in the blood is often an indicator of disease, and thus the white blood cell count is an important subset of the complete blood count. The normal white cell count is usually between 4 × 109/L and 1.1 × 1010/L. In the US, this is usually expressed as 4,000 to 11,000 white blood cells per microliter of blood. White blood cells make up approximately 1% of the total blood volume in a healthy adult, making them substantially less numerous than the red blood cells at 40% to 45%. However, this 1% of the blood makes a large difference to health, because immunity depends on it. An increase in the number of leukocytes over the upper limits is called leukocytosis. It is normal when it is part of healthy immune responses, which happen frequently. It is occasionally abnormal, when it is neoplastic or autoimmune in origin. A decrease below the lower limit is called leukopenia. This indicates a weakened immune system.
white blood cell (plural white blood cells)
