Receptors for the constant part of IgG (Fc gamma R) are implicated in the pathogenesis of a number of diseases. Children from mothers with an Fc gamma RIIIb deficiency may suffer from neonatal neutropenia due to an alloimmune reaction. Interindividual differences (polymorphisms) for a number of Fc gamma R represent risk factors for several infectious and autoimmune diseases. Immunotherapeutic use of several monoclonal antibody subclasses is affected by Fc gamma R polymorphisms. Fc gamma R can be used for cellular entrance by lymphotropic viruses (such as HIV) and appear to be involved in the pathogenesis of fulminant Dengue virus infections.