Why does anaphylaxis develop and how?

This is what an attack of anaphylaxis looks like at the cellular-molecular level

Anaphylactic shock is an exaggerated allergic response. Immune cells such as B cells, plasma cells, mast cells, basophils, and antibodies participate in anaphylaxis. Anaphylaxis generally occurs in two steps – the first exposure to an allergen results in sensitization, with subsequent exposures potentially resulting in an allergic reaction and anaphylaxis.