Fish Immune Response by Dr. B. Deivasigamani

All fish have an immune system to fight diseases, although the system is by no means as advanced as the ones found in mammals.

Pathogens can still enter the fish's body, either through physical injury or the digestive tract.
The spleen, as in other vertebrates, is the major filter of blood-borne antigens but it also performs immunopoietic functions.
https://youtu.be/oliC9TnvsQ8