Final answer:
An allergy is an overreaction of the immune system to typically harmless environmental antigens, causing symptoms like hay fever or anaphylactic shock. Anaphylaxis is a severe systemic reaction leading to life-threatening symptoms, treated urgently with epinephrine.
Step-by-step explanation:
An overreaction of the body's immune system to a foreign substance is known as an allergy. Allergies can cause a variety of symptoms, ranging from mild, such as sneezing and itchy eyes, to severe, like in the case of anaphylactic shock. Anaphylaxis is a serious, life-threatening systemic reaction that can include symptoms such as a dangerous drop in blood pressure and severe swelling that can block the airways. Allergies are caused by an immune response that is hypersensitive to environmental antigens that are typically harmless to most people.
Allergic responses are mediated by IgE antibodies produced by B cells upon first exposure to an allergen. When the system is exposed to the allergen again, these IgE antibodies interact with mast cells, causing them to release histamines and other inflammatory chemicals. This cascade of events can cause symptoms ranging from the mild, like in hay fever, to the severe, as seen in anaphylactic shock. In cases of anaphylactic shock, immediate treatment with epinephrine is critical to counteract the severe effects on blood pressure and breathing.