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[Histology](https://durgical.com/en/histology/) · [Blood and hematopoiesis](https://durgical.com/en/histology/#a-blood)

# 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

## Key points

- Red cells are anucleate biconcave discs about 7–8 µm wide with central pallor; they live about 120 days.
- Neutrophils are the most common leukocyte (roughly half or more); lymphocytes come second.
- Basophils are the rarest leukocyte (about one in a hundred or fewer); their dark granules hide the nucleus.
- Monocytes are the largest leukocyte, with a kidney- or horseshoe-shaped nucleus; in tissue they become macrophages.
- Platelets are anucleate fragments of megakaryocytes, 2–4 µm, living about 10 days.
- Reticulocytes (young red cells with ribosome remnants) reflect how fast the marrow makes red cells.

## 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.

## Memory aid

Among granulocytes, the darker the granules, the rarer the cell: pale lilac neutrophils are everywhere, orange eosinophils are few, and near-black basophils are almost never found.

## 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.

[Add to my cards (4)](https://durgical.app/estudiar/?add=/en/histology/blood-cells-on-a-smear-erythrocytes-leukocytes-platelets/)

## Related

- [Hematopoiesis and bone marrow](https://durgical.com/en/histology/hematopoiesis-and-bone-marrow/)
- [Blood vessels: artery, vein and capillary layers](https://durgical.com/en/histology/blood-vessels-artery-vein-and-capillary-layers/)
- [Microscopy and stains: H&E, PAS, trichrome, silver, immunohistochemistry](https://durgical.com/en/histology/microscopy-and-stains-h-y-e-pas-trichrome-silver-immunohistochemistry/)

## Related conditions

- [Iron deficiency anaemia](https://durgical.com/en/conditions/iron-deficiency-anaemia/)
- [Sickle cell disease](https://durgical.com/en/conditions/sickle-cell-disease/)
- [Megaloblastic anaemia (B12 or folate deficiency)](https://durgical.com/en/conditions/megaloblastic-anaemia/)
- [Thalassemias (α and β)](https://durgical.com/en/conditions/thalassemias/)

[← All of histology](https://durgical.com/en/histology/)

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](https://openstax.org/books/anatomy-and-physiology-2e/pages/18-3-erythrocytes) (OpenStax, Rice University, 2022); [18.4 Leukocytes and Platelets - Anatomy and Physiology 2e | OpenStax](https://openstax.org/books/anatomy-and-physiology-2e/pages/18-4-leukocytes-and-platelets) (OpenStax, Rice University, 2022); [Blood differential test: MedlinePlus Medical Encyclopedia](https://medlineplus.gov/ency/article/003657.htm) (U.S. National Library of Medicine, 2024).

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Representative model, for educational purposes only.

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