Leukocytes or White Blood Cells

Leukocytes or White Blood Cells
Leukocytes or white blood cells are important parts of the immune system in blood and lymphatic tissue.

Leukocytes, commonly known as white blood cells, are key components of the human immune system. Their primary function is defending the body against infectious disease and foreign invaders. These cells vary in their functions, appearances, and life cycles.

What Are Leukocytes?

Leukocytes are a diverse group of immune cells that circulate throughout the body, mainly in the blood and lymphatic system. They account for around 1% of blood volume, while red blood cells account for 40-45%. Unlike red blood cells, which lack nuclei and carry oxygen, white blood cells have nuclei and protect the body against infectious disease and foreign materials. They are essential for immune system functioning and are key in the body’s defense mechanisms against pathogens like bacteria, viruses, and fungi. Some of these cells also remove damaged or dead cells.

Leukocyte Production and Location

Bone marrow, a spongy tissue inside certain bones, is the site of white blood cell production. The name for this process is hematopoiesis. Hematopoiesis involves the differentiation of multipotent hematopoietic stem cells into various types of leukocytes. Once produced, they circulate in the bloodstream and are found in lymphatic tissue, ready to respond to any signs of infection or inflammation.

Types of Leukocytes or White Blood Cells

There are several types of leukocytes, each with unique histological appearances and functions:

  1. Neutrophils: These are the most abundant type of white blood cells in mammals. They are an essential part of the innate immune system. Neutrophils have a multi-lobed nucleus and granular cytoplasm.
  2. Lymphocytes: These cells are pivotal in the adaptive immune response. The primary types of lymphocytes are B cells, T cells, and natural killer cells.
  3. Monocytes: These are the largest leukocytes. They differentiate into macrophages and myeloid lineage dendritic cells in the immune response.
  4. Eosinophils: These cells primarily deal with parasitic infections and are also involved in inflammation and allergy responses.
  5. Basophils: The least common type of white blood cell, basophils play a role in allergic responses and parasite infections.


Neutrophils are the first responders to microbial infection. They primarily target bacteria and fungi. Their primary mechanism is phagocytosis, where they engulf and destroy the pathogens.


Lymphocytes are pivotal in the adaptive immune system. They are further divided into:

  • B cells: Produce antibodies which are used to attack invading bacteria, viruses, and toxins.
  • T cells: Divided into helper T cells (which assist in the activation of other immune cells) and cytotoxic T cells (which destroy virus-infected cells and cancer cells).
  • Natural Killer cells: Play a role in the rejection of tumor cells and cells infected by viruses.


These cells are the largest type of WBC and have a longer lifespan. They deal with cellular debris and pathogens. Monocytes differentiate into macrophages and dendritic cells which then stimulate the action of other immune cells.


Eosinophils primarily deal with parasitic infections and also play a role in the inflammatory response, especially in allergic reactions. They release enzymes and toxic proteins that are effective against parasites.


Eosinophils primarily deal with parasitic infections and also play a role in the inflammatory response, especially in allergic reactions. They release enzymes and toxic proteins that are effective against parasites.

Summary Table of Leukocytes or White Blood Cells

For review, here is a table outlining the characteristics of different white blood cells.

Type of White Blood CellPercentage in AdultsFunctionsSizeNucleus AppearanceGranule AppearanceLifetime in Body
Neutrophils40-60%Phagocytosis, first response to infection12-15 µmMulti-lobedFine granules6 hours – few days
Lymphocytes20-40%Antibody production, cellular immune response7-8 µmLarge, roundNot visibleWeeks to years
Monocytes2-8%Differentiate into macrophages/dendritic cells, phagocytosis12-20 µmKidney-shapedFine granulesDays to months
Eosinophils1-4%Combat parasites, allergic reactions12-17 µmBi-lobedLarge granules8-12 days
Basophils<1%Release histamine, involved in allergic reactions12-15 µmBi-lobedLarge, dark granulesFew hours – few days

Leukocyte Histology: How to Identify White Blood Cells

Histologists use a combination of factors, including cell size, shape, the appearance of the nucleus, cytoplasmic characteristics, and staining properties, to distinguish between different types of leukocytes (white blood cells). Here’s how these factors are applied:


Leukocytes vary in size, which is a helpful initial indicator. For example, lymphocytes are typically smaller (about 7-8 micrometers in diameter), while monocytes are larger (12-20 micrometers).


The most common staining technique used in hematology is the Romanowsky-type stain, which includes variants like Wright, Wright-Giemsa, and Leishman stains. These stains are crucial as they differentiate cells based on their affinity for acidic or basic dyes:

  • Neutrophils: Show a neutral staining, hence their name. Their granules do not strongly attract either the acidic or basic components of the stain, resulting in a pale lilac color.
  • Eosinophils: Have a strong affinity for the acidic components of the stain, particularly eosin, which stains their granules bright red or orange.
  • Basophils: Exhibit an affinity for the basic components of the stain, leading to dark blue or purple-stained granules.

Nucleus Appearance

The shape and segmentation of the nucleus are distinctive features:

  • Neutrophils: Typically have a multi-lobed nucleus, often described as segmented or polymorphonuclear.
  • Lymphocytes: Have a large, round, or slightly indented nucleus that occupies most of the cell.
  • Monocytes: Feature a large, often kidney-shaped or horseshoe-shaped nucleus.
  • Eosinophils: Usually have a bi-lobed nucleus.
  • Basophils: Also have a bi-lobed nucleus, but it’s often obscured by the granules.

Cytoplasmic Characteristics

The cytoplasm of some leukocytes contains varying types of granules, which are also distinctive. Neutrophils, eosinophils, and basophiles are granulocytes, while lymphocytes and monocytes are agranulocytes.

  • Neutrophils: Have fine granules that are lightly stained.
  • Eosinophils: Contain large, uniform granules that are easily visible.
  • Basophils: Their granules are large and dark, often obscuring the nucleus.
  • Lymphocytes and Monocytes: These are agranulocytes, meaning their cytoplasm is free of granules or contains only very fine granules not easily visible under light microscopy.

Additional Techniques

Apart from these traditional methods, modern histology and pathology also employ advanced techniques like flow cytometry and immunohistochemistry. These methods use specific antibodies to identify cell surface markers unique to different leukocytes, providing a more precise identification.

Normal White Blood Cell Count and Diagnostic Tests

A normal white blood cell count in adults ranges from 4,500 to 11,000 leukocytes per microliter of blood. However, this varies based on age, gender, and overall health. Leukocyte numbers are checked through a complete blood count (CBC) test, which is often part of a routine health examination. This test measures the number and types of cells in the blood, including red blood cells, white blood cells, and platelets.

White Blood Cell Count Disorders

Having either too many or too few leukocytes causes problems:

  • Leukocytosis: This is the condition of having too many white blood cells. It can be a response to stress, infection, inflammation, or bone marrow diseases.
  • Leukopenia: This refers to a decrease in the number of white blood cells, which increases the risk of infection. It can result from various medical conditions, including viral infections and bone marrow disorders.


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