Definition: Anaphylactin is a protein that weakens blood vessels in the skin, making it more sensitive to certain drugs or allergens. Detailed definition: An anaphylactin (also known as an anaphylatoxin) is a protein produced by immune cells called mast cells and basophils in response to exposure to allergens such as pollen, dust mites, food items, and animal dander. Its action is so strong that it leads to skin swelling, rashes, and itching, causing an immediate reaction.