Group = experimental group?

Context:

6.4.4. Selective IgA deficiency

This relatively benign genetically based immunodeficiency disease occurs among European derived populations at frequencies up to 1 per 500. It can be inherited as an autosomal dominant or recessive characteristic, but the genes involved are unknown. Most cases are asymptomatic, whereas a few have recurrent usually mild infections. Selective IgA deficiency may associate with several autoimmune diseases, usually of the thyroid-gastric group, and notably with celiac disease for which the frequency versus normal is about 15 times greater [32].