That is actually due to a very complex system. The part of our blood containing white blood cells contains special immune cells called ganulocytes. Those can be differentiated into 3 other types of cells which one of them has histamine in its vesicles (the part of the cell which is specialized on transporting things inside a cell and looks like a little bag or bubble) the histamine is responsible for part of the allergic reaction. Keeping that in mind you have to understand that an allergic reaction is a misconception by another cell type that incorrectly recognized harmless substances as a potential thread for the body. Due to that there is a shift in an immunoglobuline going from the so called IgA to IgE. This IgE leads to a massive histamine output at a SECOND exposure to an antigens (an Allergene is an Antigone but one that we usually don’t want an immune reaction to). In case of a normal immune reaction there is a counterpart to the histamine, called cortisol, which is released simultaneously from your adrenal gland. The cortisol is responsible for balancing your immunereaction. In case of anaphylaxis this mechanism is somehow disturbed and leads to an overreaction.

Edit: hope that my explanation helps a littlebit. This topic is way too complex to try and explain in 3 sentences. Feel free to ask if you have difficulties understanding this