Identification and some properties of rat secretory component.

An antiserum, prepared against partially reduced and alkylated rat milk SIgA, was shown to contain antibodies reacting with a rat milk protein of alpha-2 mobility, which possessed antigenic determinants common with SIaA, eluted from Sephadex G-200 at the same position as mammalian secretory components, and could be released from rat SIgA by mild reduction and alkylation. This protein, called rat secretory component, was also detected in saliva, tears and urine. Rat free secretory component (FSC) possessed antigenic determinants which were inaccessible in rat SIgA, as found in other species. In vitro, ra FSC combined with rat serum polymeric IgA or IgM, but not with monomeric IgA or IgG. These data emphasize the many similarities between the human and rat SIgA systems.