Peanuts, penicillin, pollen — everyone knows common causes of allergic reactions, but most do not know what is going on beneath the surface, or what actually happens to the body during a severe reaction known as anaphylaxis.

When people are exposed to an allergen, the foreign substance coming in contact with the body acts as an antigen, something which stimulates an immediate response from the immune system, Encyclopaedia Britannica explains. Because the person is hypersensitive to the substance and identifies it as a threat, the body reacts in an extreme manner. The immune system’s protective white blood cells create antibodies that attach both to the foreign substance and to other bodily cells in a way that signals them to release inflammatory chemicals like histamine and serotonin, part of its defense strategy against infections. That makes the muscles of internal organs constrict and causes other visible symptoms of allergic reactions.

“Your immune system produces antibodies that defend against foreign substances,” the Mayo Clinic says. “This is good when a foreign substance is harmful (such as certain bacteria or viruses).” But when the system overreacts to something benign, “the immune system sets off a chemical chain reaction, leading to allergy symptoms.” But these aren’t ordinary allergy symptoms; they are severe reactions.

During that severe reaction known as anaphylactic shock, the flood of chemicals from the immune system can cause a number of changes in the body. The Mayo Clinic lists a drop in blood pressure, narrowed airways, a weak pulse, flushed or pale skin, hives or a rash, nausea, vomiting, a swollen throat or tongue, and dizziness or fainting. If the person is not treated, anaphylaxis can be fatal — a victim could fall unconscious or the reaction can stop the heartbeat or cause the person to stop breathing.

Epinephrine is one treatment, and works in the exact opposite way as the anaphylaxis. Also known as adrenaline, epinephrine already occurs naturally in the body and is linked to the body’s fight-or-flight stress response. The Hormone Health Network says it dilates airways “to provide the muscles with the oxygen they need to either fight danger or flee.” It contracts blood vessels, increasing blood pressure and flow, and redirects oxygenated blood toward important muscles, like in the heart and lungs.

Encyclopaedia Britannica notes that after the adrenaline is administered, doctors may also give their patient other medications such as antihistamines — which counteract the inflammatory chemical histamine that was released earlier and which are present in many allergy medications.

But the person is not out of the woods just because the reaction appears to be over. The American Academy of Allergy, Asthma & Immunology notes that sometimes symptoms leave only to return a few hours later, “so it is important ... to remain under medical observation.”