What are Bt crystals?
Protein crystals are formed during sporulation in some Bt strains. The crystals are made up of aggregates of proteins. These crystal proteins are toxic to very specific species of insects yet harmless to humans and the natural enemies of many crop pests (benenificial insects). There are more than 150 insects that are known to be susceptible in some way to Bt.
The crystal proteins bind specifically to certain receptors in the insect's intestine. Not all insects carry the same receptors allowing for high species specificity. Humans and other vertabrates do not have these receptors in their bodies, so the toxin is unable to affect us.