Like some reactions is a phrase that can refer to a variety of different types of reactions. GUT reactions are instinctive responses that come from the GUT, or stomach area. ALLERGIC reactions are an overreaction of the immune system to a foreign substance. Chemical reactions involve the transformation of one or more substances into one or more new substances. Enzymatic reactions are biochemical reactions that involve enzymes as catalysts. Kneejerk reactions are impulsive reactions that are made without thinking.