Mast cells play what key role in initiating an immune response?

Mast cell release histamine and heparins molecules that are significant in initiating/mediating the inflammatory response. This is why mast cells are highly granulated and are categorized as granulocytes. An example of such a response is when an allergen bind to the IgE of a mast cell inducing secondary communication mechanism inside the cell leading to release of the transmitters.