What substance is released by damaged tissues and mast cells that causes capillaries to dilate?

Histamine is a significant substance released by damaged tissues and mast cells during an inflammatory response. When tissues are injured or exposed to allergens, mast cells—specialized immune cells found in connective tissues—release histamine as part of the body's defense mechanism.

The primary function of histamine in this context is to cause vasodilation, which is the widening of blood vessels. This process increases blood flow to the affected area, resulting in redness and warmth, and allows for more immune cells and nutrients to reach the site of injury or infection. Furthermore, histamine increases the permeability of capillaries, facilitating the movement of fluids and immune cells into surrounding tissues, which is essential for a robust inflammatory response.

While cytokines, antibodies, and prostaglandins also play important roles in the immune response and inflammation, they do not primarily trigger capillary dilation in the same way that histamine does. Cytokines are signaling molecules that can modulate immune responses, antibodies are produced by B cells for specific pathogen targeting, and prostaglandins are involved in various functions including pain and fever responses but are not the main drivers of immediate capillary dilation. Thus, histamine's unique role in directly causing vasodilation makes it the correct answer.