We previously demonstrated enhanced transepithelial antigen transport in the intestine of allergic rodents associated with elevated expression of the low-affinity IgE receptor CD23 on enterocytes. Here, we examined the role of CD23 in the transport phenomenon using CD23–/– mice and characterized the isoform of intestinal epithelial CD23. Jejunal segments of sensitized mice were challenged with antigen. Enhanced transepithelial antigen transport and transmucosal antigen flux were found in the intestine of sensitized CD23+/+ but not CD23–/– mice. RT-PCR showed that enterocytes expressed only the isoform b of CD23. Sequencing revealed classic and alternative CD23 b transcripts lacking exon 5 ( bΔ 5) or 6, all of which were translated into functional IgE receptors. The protein encoded by bΔ 5 but not the classic b transcript was able to mediate the uptake of anti-CD23 or IgE, whereas both CD23 proteins were internalized after binding to IgE/antigen complexes. Our results suggest that the classic and alternative forms of CD23 b display distinct endocytic properties, suggesting that they are likely to play different roles in transepithelial transport of IgE and allergens.