Why are cells washed?

When culturing cells, and particularly for immunofluorescence procedures, cells are washed with a physiological buffer solution to remove extra serum, proteins, or unbound reagents. This is to both enhance the experimental results by improving signal-to-noise ratio, and also to keep the pH of the solution stable. For most cell cultures, a standard physiological pH of 7 to 7.6 is typical. Phosphate buffered saline (PBS) is a common selection, but other buffer formulations within acceptable pH range can be used.