The metabolic relationships of the immunoglobulins in fish serum, cutaneous mucus, and bile.

Passive administration of homologous, radioiodinated, purified high m.w. (HMW) or low m.w. (LMW) serum immunoglobulin (Ig) into the sheepshead, a marine teleost, showed that the LMW Ig was neither an extracellular precursor nor a degradation product of the HMW Ig. These studies also showed that the half-lives of the HMW and LMW Ig were statistically identical, i.e., approximately 16 days. Furthermore, this work demonstrated that the Ig found in cutaneous mucus and bile were not due to transudation or active transport of either the serum HMW or LMW Ig. The implications of these later findings, coupled with previous physicochemical characterizations of the Ig in fish secretions, support the hypothesis that fish, similar to higher vertebrates, have a secretory immune system.