Complex sentence: A series of allergy injections composed of increasing concentrations of naturally occurring substances such as venom to provide excellent and usually lifelong protections against adverse effects of future insect stings: the initial injections have a very low concentration, usually 1 ppb or less.
Split into simpler sentences: A series of allergy injections composed of increasing concentrations of naturally occurring substances (such as venom) provide excellent and usually lifelong protections against adverse effects of future insect stings. The initial injections have a very low concentration, usually 1 ppb or less.