Antibodies are divided into different classes and subclasses (isotypes) according to structural differences in the constant region of the heavy polypeptide chains. The different isotypes mediate specific biologic functions that are important for the response to pathogens and in the pathogenesis of immunological diseases such as allergies. The numbers of different isotypes vary in different species, humans have 9 different Ig classes and subclasses. Of these 9, only IgM, IgG1, IgG2 and IgG3 activate the classical pathway of complement. All four IgG and both IgA subclasses, but not IgM, IgE and IgD, bind to Fc receptors on neutrophils, and induce the release of granule enzymes. All four IgG subclasses but no other Ig isotype induce serotonin release from platelets. In man, only IgE binds to the high-affinity Fc receptors (Fc epsilon RI) on mast cells and basophils and induces histamine and leukotriene release. IgE also binds to low-affinity Fc receptors (Fc epsilon RII) on lymphocytes, monocytes and eosinophils; however, the functions of Fc epsilon RII on these cells are not fully established. Monocytes from patients with atopic dermatitis that express more Fc epsilon RII than monocytes from normals do not release more LTC4 than monocytes from nonallergic healthy humans after activation with aggregated IgE. IgG and IgA are more efficient than IgE in inducing the release of mediators of inflammation from monocytes. The antibody response to certain antigens such as carbohydrates and allergens are often restricted to IgG2 and IgG1, IgG4 and IgE, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)