Hi! there are 22 amino acids that constitute millions of proteins; out of these 14 can be generated in our body but rest 8 must be supplied in the food therefore they are called essential amino acids. most animal proteins contain all essential amino acids therefore they are first-class proteins but veg proteins are often deficient in one or more amino acids and therefore they are second class proteins and need food combinations like pulse-rice, wheat-pulse etc... Lysine, leucine, isoleucin, valine, methionine, phenyl-alanine, threonine, tryptophan and Histidine (in infants) are essential amino acids...!