Final answer:
An immediate hypersensitivity or Type I hypersensitivity can occur after the initial exposure to a foreign antigen without prior sensitization. This allergic reaction is mediated by the IgE antibody class and involves the activation of mast cells and the release of chemical mediators like histamine.
Step-by-step explanation:
The immune reaction that occurs after the initial exposure to a foreign antigen without prior sensitization is called an immediate hypersensitivity or Type I hypersensitivity. This type of allergic reaction is mediated by the IgE antibody class and involves the activation of mast cells and the release of chemical mediators like histamine. Common symptoms of Type I hypersensitivity include sneezing, watery eyes, and hives.