Leucocyte Physiology
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Direct answer
One microlitre of healthy adult blood carries 4,000-11,000 leucocytes: neutrophils 40-70 per cent, lymphocytes 20-40 per cent, monocytes 2-10 per cent, eosinophils 1-6 per cent and basophils under 1 per cent. Neutrophils live only 6-8 hours in circulation, so the marrow storage pool holds roughly ten times the circulating number and can flood the blood within hours, producing a left shift with band cells when counts cross 11,000. Each class has a fixed job: neutrophils handle pyogenic bacteria by chemotaxis, opsonisation, phagocytosis and a respiratory burst; eosinophils attack parasites and moderate allergy; basophils and mast cells carry IgE-triggered histamine; monocytes become tissue macrophages; lymphocytes mount adaptive immunity.
What you must remember
- Normal total count: 4,000-11,000 per microlitre; above 11,000 is leucocytosis, below 4,000 leucopenia. The Indian viva mnemonic for the differential order is Never Let Monkeys Eat Bananas — neutrophils, lymphocytes, monocytes, eosinophils, basophils.
- Neutrophil kinetics: marrow mitotic pool, then a storage (maturation) pool about tenfold larger than the circulating pool; the marginating pool equals the circulating pool, so true body neutrophil mass is roughly double the count. Half-life in blood is 6-8 hours.
- Left shift: band cells and metamyelocytes appear in the blood when marrow demand is high — acute pyogenic infection; a marked left shift with toxic granules predicts severe bacterial infection better than the total count alone.
- Phagocytosis sequence: chemotaxis (C5a, IL-8, bacterial formyl peptides, kallikrein), opsonisation (IgG and C3b), ingestion (pseudopodia, phagosome), killing — oxidative burst via NADPH oxidase (defective in chronic granulomatous disease) and myeloperoxase-halide system.
- Eosinophils: parasitic infestation (common Indian triggers — helminths), allergy, drug reactions; count falls with stress and corticosteroids (eosinopenia is an acute-phase response). Major basic protein attacks schistosomula.
- Basophils and mast cells: Fc receptors bind IgE; allergen cross-linking releases histamine, heparin and leukotrienes — the immediate hypersensitivity axis.
- Lymphocytes: about 70-80 per cent of circulating lymphocytes are T cells (thymus-processed), the rest B and NK cells; small cells dominate the smear, and reactive lymphocytes in viral infection (infectious mononucleosis) are a classic smear finding.
- Pus is dead neutrophils: an abscess is spent neutrophils plus liquefied tissue; pyrexia with a left shift and pus points to the pyogenic pathway examiners love.
A worked interpretation of a blood count
Take a count of 17,000 with 82 per cent neutrophils, 8 per cent band forms and a toxic granulation report. Read it in kinetics language: the storage pool has been drained into the circulation (count up), immature cells have been pushed out early (bands), and the cells are actively killing (toxic granules are primary granules staining coarsely under stress). The marginating pool has also mobilised, so the demarginalisation that follows exercise, stress or corticosteroids would add to the count — which is why a count after a seizure or a steroid dose cannot be read as infection.
Contrast the viral picture: 9,000 total with 55 per cent lymphocytes, some reactive with abundant cytoplasm. Here the marrow is not flooding bands; instead lymphoid tissue is responding, and the relative lymphocytosis in a child (lymphocytes dominate physiologically up to about 4-6 years of age) makes absolute counts more informative than percentages. Add one clinical layer: corticosteroid therapy causes neutrophilia with eosinopenia and lymphopenia — a pharmacological mimic of stress that examiners quote as the steroid count pattern.
Where students slip
Two errors dominate. First, calling every raised count infection: demarginalisation after exercise, surgery, pregnancy (neutrophilia is physiological in the third trimester and in labour) and steroid therapy all raise neutrophils without bacteria. Second, confusing absolute and relative values — a total count of 3,500 with 60 per cent neutrophils is absolute neutropenia (2,100) despite a "normal-looking" differential; the risk of sepsis tracks the absolute neutrophil count, classically serious below 500. In the viva, be ready to explain why chronic granulomatous disease presents with catalase-positive staphylococci: catalase-negative organisms keep their own hydrogen peroxide, which the myeloperoxidase system can use.
Frequently asked questions
What is the normal differential leucocyte count in adults?
Neutrophils 40-70 per cent, lymphocytes 20-40 per cent, monocytes 2-10 per cent, eosinophils 1-6 per cent and basophils less than 1 per cent of a total count of 4,000-11,000 per microlitre.
Why does a left shift occur in acute bacterial infection?
The marrow storage pool, about ten times the circulating pool, releases band cells and metamyelocytes prematurely when neutrophil consumption outpaces production, so immature forms appear in the peripheral smear.
Which enzyme is defective in chronic granulomatous disease?
NADPH oxidase, which generates the oxidative burst; patients suffer recurrent infections with catalase-positive organisms such as Staphylococcus aureus because the respiratory burst cannot form superoxide radicals.
Why does eosinophilia occur in parasitic infestations?
Eosinophils release major basic protein and cationic proteins toxic to helminth larvae, and IgE-mediated mast cell activation in tissue invasion recruits them — hence high counts in filariasis and intestinal helminthiasis, both common in Indian practice.
How do corticosteroids alter the leucocyte picture?
They demarginate neutrophils (neutrophilia) while sequestering lymphocytes and eosinophils in tissues, producing lymphopenia and eosinopenia — a pattern that can mask infection on a routine count.