Immunoglobulin G antibodies are complex proteins produced by cells in the immune system, according to Johns Hopkins Medicine. These abundant and specific antibodies participate in the body's response to infection by recognizing and attacking foreign antigens.
The immune system produces three major types of immunoglobin, or Ig, explains eBioscience. Of these, IgG is the most common and occurs in four structurally-distinct subclasses. Circulating IgG may be found in the blood, lymph fluid and cerebrospinal fluid. A deficiency in IgG results in frequent infections. IgG antibodies against a particular infection are retained in the immune system's memory in order to efficiently fight any recurring future infection.