It becomes clear as you read the first chapter of this amazing book and we are given a tour of a "Hatchery" that there are five castes in this society, each named after the first five letters of the Greek alphabet: alpha, beta, gamma, delta, epsilon. It also becomes clear that they are organised in order of importance, with alphas being the most important and epsilons the least. Consider what the Director of Hatcheries says to his visitors:
"We also predestine and condition. We decant our babies as socialised human beings, as Alphas or Epsilons, as future sewage workers or future..." He was going to say future World Controllers, but correcting himself, said "future Directors of Hatcheries" instead.
As we read on it becomes clear that Epsilons are given less oxygen to ensure that they have an "Epsilon environment as well as an Epsilon heredity" ensuring that this society is not troubled by any intelligent Epsilons.
Thus the first chapter introduces us to the most radically stratified class system in the world, all organized by genetic engineering and advanced science, to create five types of humans with clearly demarcated positions and roles in society.