Pregnancy-associated alpha 2-glycoprotein (alpha 2-PAG) and IgA: their parallel distribution in the human gastrointestinal tract.

Using a combined immunoperoxidase (PAP)/direct immunofluorescence (IF) staining technique, pregnancy-associated alpha 2-glycoprotein (alpha 2-PAG) has been shown to be present in the majority of IgA-producing plasma cells in the lamina propria of both the jejunum and the colon. Both proteins have also been detected on the apical surface of columnar epithelial cells, particularly in the colon, and it is suggested that alpha 2-PAG, like IgA, may be secreted into the lumen of the gastrointestinal (GI) tract by an active transport mechanism. The concentration of alpha 2-PAG in the jejunal fluid of most subjects was found to be less than 20 micrograms/liter, although a number of subjects showed substantially elevated levels. The significance of the association between IgA and alpha 2-PAG, which is known to have immunosuppressive properties, is discussed in relation to the requirement for an efficient immunologic barrier within the GI tract.