Histology · Blood and hematopoiesis
Blood cells on a smear: erythrocytes, leukocytes, platelets !
- Cell
A drop of blood spread on a slide shows red cells, five kinds of white cells and tiny platelets, each with a look that tells you what it is.
Updated 2026-11-01
What it is
The peripheral blood smear, stained with a Romanowsky stain (Wright or Giemsa), shows the formed elements: erythrocytes, leukocytes and platelets. Recognising each cell by size, nucleus and granules is the base of haematology and of reading a complete blood count with its differential. Shape changes in red cells and the mix of white cells point to anaemias, infections and leukaemias.
Structure
- Red cells: no nucleus or mitochondria, so they rely on anaerobic glycolysis; spectrin keeps them flexible.
- Granulocytes (neutrophil, eosinophil, basophil) have lobed nuclei and specific granules; agranulocytes (lymphocyte, monocyte) do not.
- A band neutrophil has a curved, unsegmented nucleus; a few are normal, many suggest a left shift in acute infection.
The cells of a normal smear
| Cell | How it looks | Share | Main function |
|---|---|---|---|
| Erythrocyte | Pink disc with no nucleus and a pale centre; about the size of a small lymphocyte nucleus | By far the most numerous cell in blood | Carries oxygen and carbon dioxide on haemoglobin |
| Neutrophil | Nucleus with 2–5 lobes joined by thin strands; fine, pale lilac granules | The largest group: about half to two thirds of white cells | First responder; phagocytoses bacteria |
| Lymphocyte | Round, dark nucleus filling most of the cell; thin rim of blue cytoplasm | Second: about one in four or five white cells | B, T and NK cells; specific immunity and killing of infected cells |
| Monocyte | Largest cell; folded, kidney or horseshoe nucleus; grey-blue cytoplasm | A few in every hundred white cells | Leaves blood to become a macrophage; clears microbes and debris |
| Eosinophil | Usually two-lobed nucleus; large orange-red granules | A few in every hundred white cells | Defence against helminths; allergic inflammation |
| Basophil | Dense dark purple-blue granules that cover a two-lobed nucleus | The rarest: one in a hundred or fewer | Releases histamine and heparin; allergic reactions |
| Platelet | Small purple fragment with no nucleus, often in small clumps | Many per field, far more than white cells | Haemostasis: forms the platelet plug and releases growth factors |
Markers
- Wright-Giemsa
- Reticulocyte stain (new methylene blue)
- MCV
- Differential count
Clinical notes
- Iron deficiency: small, pale red cells (microcytic, hypochromic) with variable size; the most common anaemia worldwide.
- Sickle cell disease: rigid crescent-shaped red cells formed by haemoglobin S at low oxygen; they block small vessels.
- Megaloblastic anaemia (B12 or folate deficiency): large oval red cells and neutrophils with too many lobes.
- Eosinophilia in Colombia and the tropics should raise the question of intestinal helminths.
In the lab
- The automated blood count gives the totals; the smear confirms shapes, inclusions and immature cells.
- The differential reports each white cell as a percentage; absolute counts (percentage × total) are more meaningful.
Test yourself
Think of the answer, then tap the question to see it.
Which leukocyte is the most abundant in normal adult blood?
The neutrophil, which makes up about half to two thirds of white cells; it has a lobed nucleus and fine pale granules.
What is the largest white cell on a smear, and what does it become in tissues?
The monocyte, with a folded kidney-shaped nucleus; once it leaves the blood it matures into a macrophage.
Why can an erythrocyte not use the oxygen it carries?
It loses its nucleus and mitochondria while maturing, so it makes its energy by anaerobic glycolysis.
What smear findings suggest megaloblastic anaemia?
Large oval red cells (macro-ovalocytes) and hypersegmented neutrophils, from impaired DNA synthesis due to lack of B12 or folate.
Related
- Hematopoiesis and bone marrow
- Blood vessels: artery, vein and capillary layers
- Microscopy and stains: H&E, PAS, trichrome, silver, immunohistochemistry
Related conditions
- Iron deficiency anaemia
- Sickle cell disease
- Megaloblastic anaemia (B12 or folate deficiency)
- Thalassemias (α and β)
Educational content on normal microscopic structure and its clinical relevance; no substitute for studying slides or for a health professional's advice.
Sources: 18.3 Erythrocytes - Anatomy and Physiology 2e | OpenStax (OpenStax, Rice University, 2022); 18.4 Leukocytes and Platelets - Anatomy and Physiology 2e | OpenStax (OpenStax, Rice University, 2022); Blood differential test: MedlinePlus Medical Encyclopedia (U.S. National Library of Medicine, 2024).
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Representative model, for educational purposes only.