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Portrait of White blood cell

Anatomy · Cells & tissues

White blood cell

The body's mobile defenders

Blood · formed element

The cells of the immune system that circulate in the blood and can leave it to enter the tissues. Unlike red cells they keep their nuclei, and they number only about 4,000–11,000 per microlitre of blood. There are five main types: neutrophils, eosinophils and basophils (the granulocytes) and lymphocytes and monocytes (agranulocytes). They fight bacteria, parasites, viruses and abnormal cells and take part in inflammation and allergy.

How to spot them

  • ▸Larger than a red cell, with a stained nucleus and little visible colour of their own
  • ▸Neutrophil: nucleus in three to five lobes, pale pink granules
  • ▸Lymphocyte: round, dark nucleus filling nearly the whole cell, with a thin blue rim of cytoplasm
  • ▸Monocyte: the largest, with a kidney- or horseshoe-shaped nucleus
  • ▸Eosinophil: bilobed nucleus and bright red-orange granules; basophil: coarse deep-purple granules that hide the nucleus

What to know

Five types in falling order of abundance, remembered by 'Never Let Monkeys Eat Bananas'

White blood cell — neutrophil, lymphocyte, monocyte, eosinophil, basophil.

The commonest type has a nucleus of three to five lobes and is the first to arrive at an infected wound

White blood cell — the neutrophil; pus is largely dead neutrophils.

A Russian zoologist watched starfish larvae engulf a thorn and proposed cells that eat invaders, sharing a Nobel Prize in 1908 (1908)

White blood cell — Élie Metchnikoff and the theory of phagocytosis; he shared the Prize with Paul Ehrlich.

Their number in blood rises in infection and falls in some marrow diseases; normal is about 4,000–11,000 per microlitre

White blood cell — leukocytosis and leukopenia.

Some mature in the thymus, others in bone marrow, and they carry the memory of past infections

White blood cell — T and B lymphocytes.

The cell with a bright red-orange granule stain that rises in allergy and parasitic worm infection

White blood cell — the eosinophil.

The largest type, which leaves the blood to become a macrophage in tissue

White blood cell — the monocyte.

Key ideas

  • Granulocytes (neutrophils, eosinophils, basophils) are innate defenders: neutrophils engulf bacteria, eosinophils attack parasites, and basophils release histamine in allergic reactions.
  • Lymphocytes carry out adaptive immunity: B cells make antibodies, T cells kill infected cells and coordinate the response, and NK cells attack abnormal cells.

Quick-fire facts

  • •A normal count is about 4,000–11,000 per microlitre, only a fraction of one per cent of blood cells
  • •Neutrophils make up about 40–70% of the count, lymphocytes 20–40%, monocytes 2–8%, eosinophils 1–4% and basophils under 1%
  • •All arise from stem cells in the red bone marrow, and lymphocytes mature in the thymus (T cells) or marrow (B cells)
  • •Neutrophils live only hours to a few days in the blood; some lymphocytes live for years
  • •Élie Metchnikoff and Paul Ehrlich shared the 1908 Nobel Prize in Physiology or Medicine for work on immunity
  • •Leukaemia is cancer of the white cell lines; the word means 'white blood'
  • •Monocytes leave the blood and become tissue macrophages and dendritic cells
  • •The differential white count is one of the commonest blood tests

Drill it — 8 cards

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Sources