In the biology notebook I wrote right this: Histamines ⇒ a compound that is released by cells in response to injury and in allergic and inflammatory reactions, causing contraction of smooth muscle and dilation of capillaries. So, the answer is "Histamines".
Mucous is a sticky substance which traps bacteria and other pathogens. Because mucous isn't released in the blood, it's an external defense. Histamines are released in the bloodstream, making it an internal defense.
