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. The gene discussed is CD79A; the disease is purpura.