The primary structure of the immunoglobulin found in intestinal effluents most likely is the immunoglobulin Fab or F(ab') 2 fragments found in their stool [58, 202] , which nevertheless maintains its antigen-binding activity, as indicated by the correlation between appearance of the immunoglobulin and the virus-neutralizing activity observed in stool samples [58] . In adults ingesting bovine anti-Clostridium difficile immunoglobulins, toxin-neutralizing activity paralleled the bovine IgG content in ileal effluent [205] , and in stool samples [204] . A pepsin-resistant form of bovine IgG representing approximately 10% of colostral immunoglobulin has been isolated with a lectin that binds O-linked oligosaccharides [185] , indicating that some proportion of IgG in the gastrointestinal tract may remain intact.