Histamine is a chemical that is released as part of a normal immune response to an antigen. It is released by cells called mast cells. Note the vesicles containing histamine inside the mast cell.

It is also released by a type of white blood cell caused a basophil.

Histamine causes a dilation of blood vessels and also makes the walls of the blood vessels e.g capillaries, more "leaky". This results in the increased flow of the components of the immune system found in the blood to the reaction site.

An allergic reaction is when an excessive amount of histamine is released into the bloodstream usually in response that an antigen that is not particularly harmful e.g pollen, bee venom. Body cells with receptor to histamine respond to give a number of symptoms associated with allergic reaction e.g itching, running nose and eyes (to try and remove the perceived pathogen), skin rashes. In serious overreactions airways can narrow, excess mucus is produced in the airways, the tongue, lips, face can swell up (in response to excessive leakage of tissue fluid from blood vessels) and this is described as anaphylactic shock. This needs to be treated immediately with antihistamine drugs.