The transfer of serum IgG1 antibody into the gastrointestinal tract in newborn calves

Transfer of functional blood IgG1 to the gastrointestinal tract was measured in neonatal calves. Radiolabelled immunoglobulin G1 (IgG1) anti-DNP antibody was administered to 2 day old calves by intravenous injection. The serum clearance rate was measured and was compared to the rate of protein-bound (125)I excretion in the feces over a 10 day period to determine the importance of transfer to the gastrointestinal tract as a mechanism of serum IgG1 clearance. The amount of protein-bound and DNP-binding (125)I present in the gastrointestinal tract of 10 day old calves at necropsy was also measured. Fecal excretion of protein-bound (125)I accounted for 32% of the serum (125)I-IgG1 clearance. Protein-bound (125)I was present in the gastrointestinal tract at necropsy in amounts estimated to account for 68% of the total (125)I-IgG1 clearance, and retained 65% of the DNP-binding ability of the original antibody. The discrepancy between the fecal excretion (32% of total IgG1 clearance) and the GI clearance estimated from protein-bound (125)I in the gut (68% of total IgG1 clearance) is explained in part by IgG1 proteolysis occurring after transfer to the gastrointestinal tract but before fecal excretion. These results indicate that transfer to the calf gastrointestinal tract accounts for most IgG1 clearance in young calves, and that the intestinal antibody retains antigen binding function and may contribute to intestinal immunity.