A reactive pia arachnoid mesothelial cell as noted by the darker cytoplasm is present in this field. Reactive cells are a common finding in cytospin smears from spinal fluid samples and are sometimes difficult to distinguish from tumor cells. Mesothelial cells are usually interspersed among the other cells, rather than appearing in clumps. They have a single distinct nuclei that may be eccentric.

The macrophages (histiocytes) are seen next to the mesothelial cell. Macrophages are distinguished from circulating monocytes by the irregular appearing cytoplasm. Bacteria, red cells or other debris can often be seen in the cytoplasm of macrophages.