Final answer:
The statement is true; mast cells contain histamine and heparin, which they release upon encountering an allergen, leading to allergic reactions, including anaphylactic shock in severe cases.
Step-by-step explanation:
The statement that mast cells mediate allergic reactions in local tissues is true. Mast cells, located in connective tissues, are integral to the immune system and play a pivotal role in allergic reactions. They contain cytoplasmic granules rich in inflammatory mediators such as histamine and heparin. When a person with sensitized mast cells (due to prior exposure to an allergen) encounters the allergen again, the mast cells degranulate, releasing histamine and causing symptoms of allergy like sneezing, redness, and swelling. The severe allergic reaction called anaphylactic shock involves symptoms like airway constriction and plummeting blood pressure and requires immediate medical attention.