Detection of specific and 'constitutive' antibody secreting cells in the gills, head kidney and peripheral blood leucocytes of dab (Limanda limanda)

The relative immunological importance of the gills of fish was investigated in terms of antibody production by enumerating antibody secreting cells (ASC) in the gills, head kidney and blood of dab (Limanda limanda) using the ELISPOT assay. The contribution of 'constitutive' ASC in the gill appeared more substantial than that of elicited specific ASC. The gills were found to contain a mean (±SD) of 4227 ± 1029 'constitutive' Asc/10⁶ cells which was fewer than the head kidney which contained a mean (±SD) of 15617 ± 3723 'constitutive' ASC/10⁶ cells but more than peripheral blood leucocytes which contained a mean (±SD) of 2650 ± 212 'constitutive' ASC/10⁶ cells. The number of specific anti-human gamma globulin (HGG) ASC following parenteral or oral administration of HGG was also determined. Anti-HGG ASC were detected in all three tissues following parenteral immunization, peaking simultaneously, 4 weeks post-immunization. The strongest response was found in the head kidney. After oral immunization, responses were much weaker; again the head kidney was the most active but the gill response was barely detectable. These data were complemented by measurement of specific antibody in the serum by ELISA. Serum antibody titres following immunization were found to correlate closely with the number of specific ASC in the head kidney following parenteral immunization whereas serum antibody titres after oral administration of antigen most closely followed the number of specific ASC in the blood. In the light of these data it is suggested that the primary immunological role of the leucocytes in the gill may be in the earliest stages of defence against infection.