What type of allergic reaction may occur without any prior sensitization?

Anaphylactic reactions can occur in response to a substance that the body has not previously been sensitized to, which distinguishes them from other types of allergic reactions that typically require prior exposure to an allergen. Anaphylaxis is a severe, potentially life-threatening systemic response that can happen rapidly after exposure to an allergen, including foods, insect stings, or medications.

In anaphylaxis, the immune system can react to a substance it has not encountered before because some people may have a genetic predisposition or a heightened immune system response. This can lead to the immediate release of histamine and other chemicals, resulting in symptoms like swelling, difficulty breathing, and a drop in blood pressure.

Delayed hypersensitivity reactions, on the other hand, generally require prior sensitization because they are mediated by T cells rather than antibodies. Autoimmune responses involve the immune system attacking the body’s own tissues, which also doesn't pertain to an immediate allergic reaction. IgE-mediated reactions typically mean that there has been an initial exposure to an allergen that leads to an IgE antibody response, thereby requiring prior sensitization as well. The unique characteristic of anaphylactic reactions allows them to occur without prior exposure, making them particularly notable in emergency medical situations.