Final answer:
An allergic reaction is a hypersensitivity to typically harmless substances, with 'allergic' being the correct term to fill in the blank. This immune response involves IgE antibodies and can range from mild symptoms to severe anaphylactic shock.
Step-by-step explanation:
An allergic reaction is defined as a hypersensitivity to substances that normally pose no serious risk to the body, such as pollen, food, or animal hair. The correct term to fill in the blank is 'allergic,' making the full statement: 'An allergic reaction is defined as a hypersensitivity to substances that normally pose no serious risk to the body.'
The primary immune response in such a scenario involves the production of IgE antibodies upon the first encounter with the allergen. The sensitization of mast cells by these IgE antibodies prepares the immune system to react immediately upon subsequent exposures to the allergen. Reactions can range from mild, such as sneezing and itchy eyes, to severe, as in anaphylactic shock, which can be life-threatening if not promptly treated with epinephrine.