Monocytes function
Monocytes are the precursors of macrophages . Monocytes function by responding to infection. Thus monocytes like granulocytes play an important role in non-specific immunity and the inflammatory response.
However monocytes are much more phagocytic than granulocytes.
Monocytes origin
Monocytes are also present in large numbers in the kidney, suggesting that they are derived from stem cells in this organ.
Monocytes occurrence
In fish, the blood contains variable numbers of monocytes, but generally the numbers are low.
For example in Plaice they account for only 0.1% of the total number of leucocytes. In Catfish however they are more numerous,making up 1-8% of the leucocytes.
Monocytes are considered to be quite rare in the circulating system in Rainbow Trout (Oncorhynchus mykiss).