[The significance of determining humoral defense factors: Ig isotypes, specific antibodies and complement]

The diagnosis of an immunodeficiency is made after detailed clinical and laboratory investigations. To guide these investigations the Dutch Working Group on Immunodeficiencies had made a protocol for evaluating the functioning of the immune system of patients. The protocol is based on the report 'Immunodeficiency' from a WHO scientific group. This paper discusses the value of laboratory investigations on the following aspects of the immune system: immunoglobulins and their subclasses, primary and secondary antibody responses in vivo, and the complement system. The evaluation of these components of the defense system can firmly establish possibly occurring severe immunodeficiency states. Some cases of chronic and/or recurrent infections, however, remain enigmas that have to be resolved by future investigations on the interactions of infectious agents with the immune system.