Final answer:
Type I hypersensitivity reactions, like atopic dermatitis, involve IgE antibodies which bind to mast cells and cause them to release histamines and other substances that lead to allergic symptoms upon secondary exposure to the allergen.
Step-by-step explanation:
Type I hypersensitivity reactions, such as atopic dermatitis, involve the action of IgE antibodies acting on mast cells. When an individual is first exposed to an allergen, IgE antibodies are synthesized by plasma cells in response to the harmless antigen. These IgE molecules then bind to mast cells. Upon secondary exposure to the allergen, these sensitized mast cells release histamines and other modulators that cause the symptoms of an allergy. This process is initiated by the binding of the allergen to the IgE bound to the mast cells, resulting in degranulation.
Type II hypersensitivity reactions, on the other hand, involve IgG and IgM antibodies directed against cellular antigens, leading to cell damage mediated by other immune system effectors. In contrast, type I hypersensitivity is mediated by IgE and is characterized by the immediate release of inflammatory molecules such as histamines from mast cells.