Granulocytes are involved in non-specific defence mechanisms, i.e. they respond to the presence of foreign material in the body but do not recognise specific antigens. These cells migrate to parts of the body where invasion occurs and destroy the foreign particles by phagocytosis or by direct killing known as the cytotoxic response. This process is termed the inflammatory response.
There is some evidence that fish lymphocytes are differentiated into at least two functional sub-populations with functions similar to those of B and T lymphocytes in mammals. The function of lymphocytes is to mediate the humoral and cellular immune response.On making contact with foreign material (antigen) there is a proliferation of lymphocytes, which then secrete large quantities of immunoglobulin antibody.