A broad classification of dysgammaglobulinemias characterized by low or undetectable serum levels of one of the five immunoglobulin classes. Deficiencies of immunoglobulins present variably according to isotype. Selective deficiencies may be caused by decreased or inefficient production from progenitor B cells without any corresponding decreases in the other isotypes. The clinical course and prognosis is dependent upon the severity of the selective deficiency and associated morbidity.