Plants are made up of thousands of chemicals, and Neem is no different in this respect. However, scientists are particularly interested in one family of chemicals particular to Neem, and responsible for its' amazing properties.
These are the 'tetranortriperpenoids' or more specifically 'liminoids' - similar to steroids.
About 40 of these chemicals act together, producing a 'synergistic' or 'enhanced' affect.
The most active and well-studied of these is 'Azadirachtin'.
As scientists continue to fully unravel the mystery of Neem, they have discovered that although Neem terpenoids are present in almost all parts of the plant, the site, synthesis and accumulation of these chemicals occurs in the 'secretory cells'. These cells are most abundant in the seed kernels.

The following are the most abundant and well-studied chemicals in the Neem Tree, along with their recognised effects:
Nimbin - anti-inflammatory, anti-pyretic, antihistamine, anti-fungal
Nimbidin - antibacterial, anti-ulcer, analgesic, anti-arrhythmic, anti-fungal
Nimbidol - antitubercular, anti-protozoan, anti-pyretic
Gedunin - vasodilator, anti-malarial, anti-fungal
Sodium Nimbinate - diuretic, spermicide, anti-arthritic
Quercetin - anti-protozoal, antioxidant, anti-inflammatory, antbacterial
Salannin - repellent
Azadirachtin - repellant, anti-feedant, anti-hormonal