Why do allergies exist? why do some people have allergies while others do not? why do some allergies develop later in life?

Immune system: It is the immune system, and it is genetically determined, we are all born with the propensity to be allergic to something at some point in our lives, which might not come at all, may be because we don't become exposed to what we would react to, anyways, our immune sydtems can tolerate certain exposures to certain limits, people differ in that threshold, hence react differently,