The criteria include the mandatory presence of purpura or petechiae predominantly in the lower limbs without thrombocytopenia or coagulopathy, along with at least one of the following: abdominal pain, arthritis or arthralgia, renal involvement with proteinuria or hematuria, and histological evidence of leukocytoclastic vasculitis or proliferative glomerulonephritis with predominant IgA deposition. This evidence concerns the gene CD79A and arthritic joint disease.