What Are Leukocytes (White Blood Cells)?
Leukocytes are immune cells that protect the body against infections by fighting pathogens through direct and indirect mechanisms, comprising several distinct subtypes including neutrophils, lymphocytes, monocytes, eosinophils, and basophils. 1
Core Function and Role
- Leukocytes form the immune system and are responsible for defending against foreign elements including viruses, bacteria, parasites, and fungi. 2, 3
- They provide both active immunity and promote the development of immunocompetence through various mechanisms. 1
- Circulating leukocytes represent less than 5% of the body's total leukocyte pool, meaning blood levels do not necessarily reflect tissue-specific immunological reactions. 4
Major Leukocyte Subtypes
Granulocytes (contain granules in cytoplasm):
- Neutrophils: Most abundant type; primary responders to bacterial infections through phagocytosis and respiratory burst mechanisms 1, 5
- Eosinophils: Involved in allergic reactions and parasitic infections 1, 2
- Basophils: Least common type; involved in allergic and inflammatory responses 1, 2
Agranulocytes (lack visible granules):
- Lymphocytes: Include T cells and B cells; critical for adaptive immunity 1, 6
- Monocytes: Differentiate into macrophages and dendritic cells; involved in phagocytosis and antigen presentation 1, 7
Physical Characteristics
- Normal concentration in blood: 4,000-11,000 cells/mm³ (4-11 × 10⁹/L) 1
- Size range: 10-24 micrometers in diameter 1
- Density: 1.067-1.1 g/mL 1
- Shape: Spherical 1
- Unique molecular identifier: CD45 (leukocyte common antigen) 1
Normal Distribution of Subtypes
Each leukocyte type constitutes a different proportion of total white blood cells, with specific reference ranges:
- Monocytes normally comprise 4-10% of total leukocytes 7
- The relative proportions of each subtype provide diagnostic information about infections, inflammatory conditions, and hematologic disorders 2, 6
Clinical Significance
- Leukocyte counts increase in response to infections, with concentrations rising when either the host or affected tissue has an infection. 1
- The specific pattern of elevation (which subtypes are increased) helps distinguish between bacterial infections (neutrophil predominance), viral infections (lymphocyte predominance), allergic reactions (eosinophil elevation), and other conditions. 2
- Leukocyte functional defects define specific disease entities including chronic granulomatous disease (defective respiratory burst), lazy leukocyte syndrome (chemotaxis defects), and various immunodeficiency disorders. 5
Important Physiological Variations
- Leukocyte counts exhibit approximately 10% diurnal variation throughout the day, which is a normal biological phenomenon. 8
- Acute exercise, emotional stress, and physical stress cause transient elevations through catecholamine and cortisol release. 8, 4
- Age, sex, nutritional status, and medications (particularly corticosteroids and lithium) significantly affect leukocyte counts. 8, 2
Key Clinical Pitfall
A single leukocyte count should not be over-interpreted without clinical context, as transient elevations occur with exercise, stress, diurnal variations, and laboratory processing factors. 8 Serial measurements correlated with clinical symptoms provide more reliable assessment than isolated values. 8