Leukocyte Classification: A Guide to White Blood Cell Types and Functions

Overview

Leukocytes (WBCs) are nucleated cells of the immune system. Normal count: 4,000–11,000/µL. Classified by cytoplasmic granules and nuclear morphology.

Granulocytes (Polymorphonuclear)

Neutrophils (50–70%)

• First responders; phagocytosis, NETosis, degranulation.
• Short-lived (hours–days); "left shift" = band forms in acute infection.

Eosinophils (1–4%)

• Parasite defense (helminths); allergic inflammation (IL-5 driven).
• Major basic protein, eosinophil peroxidase.

Basophils (<1%)

• IgE-mediated immediate hypersensitivity; histamine, heparin.
• Similar to tissue mast cells.

Agranulocytes (Mononuclear)

Lymphocytes (20–40%)

• B cells: Antibody production, memory, antigen presentation.
• T cells: CD4+ helper (Th1, Th2, Th17, Tfh, Treg), CD8+ cytotoxic, γδ T, NKT.
• NK cells: Innate cytotoxicity (missing self), ADCC.

Monocytes (2–8%)

• Circulate 1–3 days → tissues → macrophages/dendritic cells.
• Phagocytosis, cytokine production, antigen presentation, tissue repair.

Clinical Utility

Differential count + morphology (toxic granulation, Döhle bodies, atypical lymphocytes) narrows diagnosis: bacterial vs. viral, leukemia, drug reaction, chronic inflammation.