--IVIg is not really an immune booster--it is an immune regulator, trying to displace autoantibodies that attack individual tissue with calmer, more "normal" immunoglobulins that only attack invading pathogens and not "self". This is, of course, why IVIg is used in autoimmune conditions: to calm down the immune response.

I too suspect that the IVIg is not the causative agent of the recurrent psoriasis. There may be many other factors, including the phenomenon that many immune modulating therapies lose some effectiveness over time as the body processes upregulate (the same reason why often in time one needs bigger doses of painkillers to combat pain of a given intensity). I suspect, since many autoimmune conditions wax and wane and "flare" in response to stress and environmental conditions, that something else was occurring, though it may be very difficult to determine just what.