Variable numbers of lymphocytes can be seen in both serous and synovial fluids and may exhibit a reactive morphology depending on the cause of the effusion. Plasma cells may also be seen but are not a frequent finding.
Plasma cells are indicated by the arrows. They contain large amounts of basophilic cytoplasm and there is a noticeable clearing adjacent to the nucleus. Notice the dense and clumped chromatin pattern. Some of these cells have a hint of cytoplasmic vacuolation that can be prominent storage vacuoles.