What is the basis for all of the symptoms associated with an allergic reaction?

Although allergic reactions are a complex web of different interactions of the various cells in the human body, it can be simplified down to pinpoint histamine as the main culprit of almost all the symptoms related to allergic reactions. This is also the main reason why a lot of the medications for allergies are called anti-histamines.

Histamine is a biochemical compound released by different cells in the human body depending on the location of stimulus. Primarily histamine is released by mast cells and basophils (this is a type of white blood cell) in response to pathogens and/or allergens. It targets different parts of the body such as nerve endings, blood vessels, etc.

Histamine is known to play various roles in the process of inflammation. It is a known vasodilator, which means it induces the blood vessels to expand. When this happens, blood pressure in the body drops. When blood pressure drops, not enough blood can supply the different organs. This is a key process as to why patients go into anaphylactic shock.

Other popular effects of histamine include inducing itching in the body and urticaria. Constriction of the bronchus and bronchial smooth muscles in the pulmonary system, explaining why patients experiencing allergies cannot breathe. And also some studies indicate stimulation of gastric cells.

The answer is observations and interpreting.