HIGH-AFFINITY SERUM-DERIVED FAB FRAGMENTS AS ANOTHER SOURCE OF ANTIBODIES IN THE GUT LUMEN OF BOTH NEONATES AND ADULTS

The authors have investigated the presence of serum-derived immunoglob ulin G (IgG) fragments in the human intestine at various ages, these f ragments possibly representing another source of antibodies in additio n to secretory IgA (SIgA). Fab fragments of the gamma isotype were fou nd to be the major molecular form of immunoglobulins in the meconium ( median value: 3.7 mg/g of stools), as compared with Fab alpha (75 mu g /g) and IgM (2.6 mu g/g). These fragments provided by molecules of the maternal serum displayed a strong antibody activity to the tetanus to roid and were also found in the stools of 1-week-old babies fed with f ormula milk. The release of serum antibodies into the digestive lumen occurs largely via hepatobiliary secretions, as suggested by the prese nce of IgG antitoxins in the bile of children operated on extrahepatic biliary atresia. In adults, the Fab antitoxins were also detected in most stool extracts. Affinity of these molecules was found to be simil ar to that of their serum counterpart with a Ka of 2.1 x 10(10) M(1) ( median value). These mucosal antibody fragments, associated with the n ormal pathway of serum IgG catabolism, could provide additional immune defences against pathogens, and be of importance to supplement an imm ature or deficient secretory immune system.