Assertion: Histamine is related with inflammatory and allergic reactions.
Reason: Histamine is a vasodialator.

The correct option is A Both assertion and reason are true and the reason is the correct explanation for the assertion.
The histamine reaction in relation to the immune response (in both allergic reactions and immune reactions) serves two main functions: 1. It causes what is called “vasodilation.” Vasodilation occurs when the muscular walls that surround blood vessels are relaxed, causing the interior of the blood vessel to widen. 2. It induces fluid secretion at the site of infection. Both of these responses are an important part of the inflammatory response. Vasolidation allows white blood cells to move easily to the site of infection. Fluid secretion is important in ridding the body of infectious agents or allergens.