Immunoglobulin E is a type of antibody produced by IgE plasma cells. These are specialized B-cell lymphocytes that make free-floating antibodies for what is termed humoral resistance. IgE is peculiar for several reasons. It is not made to be specific against only one antigen like other gamma globulins, but instead can bind with a number of dangerous proteins. Further, IgE travels to mast cells, sticks to their surfaces, and when antigens get stuck to the IgE, the mast cells secrete inflammatory compounds like histamine. Since IgE is a generalist, coded for a number of potential toxins, not just a single substance, it can decide that Juniper pollen and cat dander are antigens...and you have an allergy. Elevated production of IgE is often inherited, which is why allergies run in a family-and why, once you have an allergy, the mast cells and IgE can decide that, for the duration, a whole bunch of other stuff causes hypersensitivity reactions, stuff that wouldn’t normally bother you without an ongoing allergy.
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