What chemical do mast cells secrete?

Mast cells play a crucial role in the immune system, particularly in allergic reactions and inflammation. They are specialized cells found in connective tissues and are known for their ability to store and release various chemical mediators. The primary substance that mast cells secrete is histamine, a biogenic amine that contributes significantly to the inflammatory response.

When mast cells degranulate in response to an allergen or injury, they release histamine, which increases the permeability of blood vessels, allowing immune cells to access tissues more easily. This process is essential in the body's defense mechanism against infections and plays a key role in the symptoms experienced during allergic reactions, such as redness, swelling, and itching.

Other options, while they may have important physiological roles, are not secreted by mast cells. For example, serotonin acts mainly in the central nervous system and gastrointestinal tract, adrenaline is associated with the adrenal gland and the fight-or-flight response, and insulin is produced by pancreatic β-cells to regulate blood sugar levels. Therefore, histamine is the correct answer, as it directly relates to the function of mast cells in immunological responses.