Reactive mesothelial cells can be found when there is an infection or an inflammatory response present in a body cavity. This condition can be due to the presence of a bacterial, viral, or fungal infection. It can also be the result of trauma or the presence of metastatic tumor.
Reactive mesothelial cells tend to come in clusters and clumps and have a more washed-out cytoplasm in body fluids. Notice in the image on the right how indistinct the cytoplasmic borders are in this clump compared to normal mesothelial cells. The wide separation of the nuclei and the well-defined nucleoli help to identify these as reactive mesothelial cells. However, if there is any doubt, the smear should be sent for hematology or pathology review.
Note: It is not uncommon for macrophages to be mixed into a reactive mesothelial cluster.