Final answer:
Basophils, part of the white blood cell family, are responsible for the release of heparin, which is an anticoagulant aiding in immune responses.
Step-by-step explanation:
The type of white blood cells believed to release heparin are basophils. Basophils are a part of the granulocyte family, the least numerous of the white blood cells and known for playing a role in the body's immune responses. They carry granules that store histamines, which aid in the body's inflammatory response by dilating blood vessels and increasing permeability. Heparin, a short-acting anticoagulant, is another component stored in basophils' granules and released to inhibit blood clotting, which facilitates the movement of leukocytes to areas of infection or tissue injury.