Final answer:
Type I Hypersensitivity reactions are characterized by antigen cross-links with IgE on mast cells and can range from minor reactions to potentially fatal episodes.
Step-by-step explanation:
Type I Hypersensitivity reactions are characterized by the cross-linking of antigens with immunoglobulin E (IgE) on the surfaces of mast cells, leading to degranulation and the release of inflammatory mediators. While the initial exposure to an allergen sensitizes the immune system and primes mast cells, subsequent exposures result in immediate hypersensitivity reactions through this IgE and mast cell interaction. Additionally, these reactions can range in severity from minor, such as hives, to potentially fatal responses like anaphylaxis.