After a penicillin injection, a patient rapidly develops respiratory distress, vomiting, and hives. This reaction is primarily mediated by?

The reaction described is primarily mediated by IgE antibodies. In an allergic response to penicillin, the immune system mistakenly identifies the drug as a harmful substance and produces IgE antibodies specific to it. Upon subsequent exposure to the same allergen (in this case, penicillin), these IgE antibodies bind to mast cells and basophils, leading to the release of histamines and other chemical mediators. This releases a variety of symptoms, including respiratory distress, vomiting, and hives, all of which are characteristic of a Type I hypersensitivity reaction.

IgM, IgG, and IgA do not primarily mediate acute hypersensitivity reactions. IgM is typically the first antibody produced in response to an infection, and it plays a role in classical complement activation. IgG, although it can lead to allergic reactions in certain contexts (like delayed hypersensitivity), is predominantly involved in opsonization and pathogen neutralization. IgA is primarily found in mucosal areas and does not play a direct role in the rapid allergic reactions seen in this scenario. Thus, IgE is the correct answer as it is the antibody class directly involved in mediating the rapid onset of symptoms following exposure to allergens like penicillin.