Allergic reactions are primarily caused by what chemical in the body?

Allergic reactions are primarily caused by histamines, which are chemicals released by certain cells in the body during allergic responses. When the immune system identifies a harmless substance, like pollen or pet dander, as a threat, it triggers a series of reactions that lead to the release of histamines. These chemicals play a critical role in the inflammatory response by causing blood vessels to expand, leading to symptoms such as itching, swelling, and redness. This response aims to help the body eliminate the allergen, but in some individuals, it can result in excessive or unwanted reactions.

The other options involve different functions: cortisol is a steroid hormone that helps manage stress and inflammation; adrenaline prepares the body for a fight-or-flight response during stressful situations; and serotonin primarily regulates mood and, to some extent, the sleep-wake cycle. None of these are directly responsible for the immediate physiological responses seen in allergic reactions, as histamines are specifically tied to that process.