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Atypical Lymphocytes vs. Monocytes

The CSF smear shown on the right was taken from a patient with partially treated bacterial meningitis. Neutrophils are the predominant cell type; however, there are also monocytes and a few atypical lymphocytes.

The atypical lymphocytes (blue arrows) have a more generous and basophilic cytoplasm than the normal, smaller lymphocytes. They also have a more regular nuclear shape compared to the monocytes (red arrows). The lymphocytes also have a more regular cytoplasmic border without the blebs and pseudopods present in the monocytes. Another key feature in the differentiation of these two cell types is the vacuolation present in the cytoplasm of the monocytes, which is not present in the lymphocytes.