serum immunoglobulin G subclass level serum immunoglobulin G1 level 2014-01-27T14:57:46Z CMO:0002115 The amount in a specified sample of serum of the G1 subclass of immunoglobulin G (IgG), the most abundant class of antibodies found in blood serum and lymph. IgG antibodies are active against bacteria, fungi, viruses, and foreign particles, and trigger action of the complement system. IgG subclasses differ in the structure of their hinge regions. IgG1 is the most abundant subclass in serum, and is able to activate complement and bind to the Fc receptor on phagocytic cells. clinical_measurement_ontology jsmith serum IgG1 level