Protiens can come from all kinds of things - from soy beans to milk to coconuts to silk and hooves.  This is listed as an "iffy" ingredient because a fairly high percentage of curlies have hair that reacts badly to it.  The normal "bad reaction" means that once its applied, hair becomes dry and/or rough.  In many cases, the sensitivity to it is limited to a particular type - such as silk or whey.