Test for leukocytoclastic vasculitis

I had a leukocytoclastic vasculitis diagnosed last year (confirmed with biopsy). At that time I had quite a few lesions on my legs. Since then, from time to time, I have had much milder outbreaks--with only a couple of lesions on my legs. My specialist wants me to have a immunofluorescence test to look for IgA. Why is that important if my outbreaks are so comparatively mild? And, how common is this type of skin condition?