Complex sentence: There appears to be a strong hereditary component, presumably certain genes coding proteins involved in the normal immune response mechanism i.e Human leukocyte antigen, although environmental factors have also been implicated.
Split into simpler sentences: There appears to be a strong hereditary component. The hereditary component is presumably due to certain genes coding proteins involved in the normal immune response mechanism i.e Human leukocyte antigen, although environmental factors have also been implicated.