Basophil granulocytes and tissue mast cells and their mediators play a role in the pathogenesis of several immune and inflammatory disorders. Human basophils and mast cells (FcepsilonRI+ cells) can be activated through immunological interaction with the IgE-FcepsilonRI network. FcepsilonRI+ cells can be triggered by cross-linking between the Fab portions of IgE and multivalent antigens (direct anaphylaxis). 'Reverse type' anaphylaxis can occur through three distinct mechanisms: antibodies against the Fcepsilon portion of IgE (anti-IgE), antibodies against epitopes of the alpha chain of FcepsilonRI (anti-FcepsilonRIalpha) and anti-IgG acting on IgG-IgE complexes bound to FcepsilonRI. Anti-IgE autoantibodies are occasionally present even in normal donors and more frequently in a variety of allergic (chronic urticaria, atopic dermatitis and bronchial asthma) and autoimmune disorders (rheumatoid arthritis, lupus erythematosus and systemic sclerosis). IgG anti-IgE from a small percentage of patients induces the release of mediators from human FcepsilonRI+ cells. Some of the anti-IgE autoantibodies present in allergic patients are non-anaphylactogenic, thus representing a possible protective mechanism preventing the association of IgE with FcepsilonRI. Anti-FcepsilonRIalpha autoantibodies also occur in a significant percentage of patients of chronic urticaria and probably non-allergic asthma and some autoimmune diseases. Although anti-IgE and anti-FcepsilonRIalpha autoantibodies, present in a percentage of patients with immune disorders, are relevant to the pathogenesis of these conditions, much remains to be learnt about their immunochemistry, their prevalence and precise role in various inflammatory diseases.