CIE A Level Biology (9700) 2019-2021

Revision Notes

8.1.4 Cells of the Blood

Cells of the Blood

  • Blood is a tissue composed of a number of important specialised cells
  • Red blood cells, monocytes, neutrophils and lymphocytes all have distinguishable structures which enable them to be recognised on microscope slides, in photomicrographs and in electron micrographs

Red blood cells

  • There are approximately 5 million red blood cells per mm3 of blood
  • Red blood cells contain haemoglobin, a protein with a quaternary structure that contains haem iron groups which can bind reversibly to oxygen
  • Distinctive features of erythrocytes when viewed under a microscope, are their distinctive biconcave disc shape (caused by their lack of nucleus)


  • Monocytes are identifiable by their size – they are the largest of the leukocytes and have a nucleus shaped like a kidney or a bean
  • The nucleus of monocytes tends to appear lighter after staining than other leukocytes
    The nucleus should appear a light blue colour, while the chromatin inside is distinct and fine


  • Neutrophils are distinguished by their multi-lobed nuclei
  • Up to 70% of all leukocytes are neutrophils – this makes them easy to spot on a micrograph
  • The granules of neutrophils typically stain pink or purple-blue


  • Lymphocytes are small leukocytes that are identifiable by their very large nuclei, which typically stains a dark colour
  • Lymphocytes constitute around 20-25% of all leukocytes
  • Lymphocytes are around the size of red blood cells

Exam Tip

When looking at micrographs, ensure you distinguish between the kidney-shaped nucleus of a monocyte and the multi-lobed nucleus of a neutrophil, as these can appear similar at first. As with all things, practice is key here!

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