The understanding of allergy is growing exponentially with the ongoing expansion in our knowledge of the immune system. The immune system is the capacity to distinguish harmful non-self molecules from self-molecules, a characteristic that exists in a delicate balance between tolerance to self and response or rejection of non-self. This system is focused on host defense and is composed of specific cellular and protein components that develop and function in a highly complex manner, in order to neutralize or destroy dangerous non-self while preserving self.