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.