The document discusses hypersensitivity reactions and their classification. Immediate hypersensitivity reactions are IgE antibody and mast cell mediated responses to antigens. IgE antibodies bind to receptors on mast cells and basophils, initiating the reaction upon re-exposure to the antigen. Mast cells, basophils, and eosinophils play essential roles in allergic inflammation. The document also discusses the roles of these cells and antibodies in initiating and propagating immediate hypersensitivity reactions.