How do barbiturates cause steven johnson syndrome?

Stevens-Johnson : Sjs is an immune mediated reaction where your body is tricked into thinking of itself as the enemy. Some people have livers that can't metabolize barbiturates well, they are "slow acetylators". This leads to an accumulation of metabolite byproducts that may bind to normal tissue ("haptens") and trigger a destructive immune response against the skin ("keratinocytes" in the "epidermis").