Blastocystis is a protozoan thought commensal (normal flora) but actually shown to cause disease in humans. Now a particular subtype is linked to urticaria/hives.
Acta Trop. 2015 May 11;148:156-161. doi: 10.1016/j.actatropica.2015.05.004.
Blastocystis and urticaria: Examination of subtypes and morphotypes in an unusual
Casero RD(1), Mongi F(2), Sánchez A(3), Ramírez JD
Nonspecific gastrointestinal symptoms/IBS have been found related to blastocystis hominis before:
Irritable Bowel – Possible Missed Protozoan Disease With Treatment
specific allele found in chronic uriticaria.
Comment – if you have an irritable bowel and hives, get stools tested for blastocystis though some labs ignore them in exams because the could be “normal” . Specifically have doctor check for them in lab req.