Participants who tested Blastocystis-positive (combined IBS-P and HC-P subgroups) had a significantly lower serum IgA level (1.59 ± 0.54 g/L) than their Blastocystis- negative counterpart (combined IBS-N and HC-N) subgroups (2.65 ± 1.41 g/L) (t(78) = 4.06, p < 0.001). This evidence concerns the gene CD79A and irritable bowel syndrome.