This marker helps to evaluate sinopulmonary infections, asthma; immunotherapy hyposensitization; and allergies.

IgG immunoglobulins are composed of four subtypes named IgG1, IgG2, IgG3 and IgG4. Each subclass is present in the serum in different concentrations, varies with age, and has different roles for immune response. Abnormal levels of one or more subclasses may be associated with certain conditions.

IgG1:

IgG1 deficiencies are associated with EBV infections

IgG2:

IgG2 deficiencies are associated with sino respiratory infections and infections with encapsulated bacteria, 

IgG3:

IgG3 deficiencies are associated with sinusitis and otitis media, 

IgG4:

IgG4 deficiencies are associated with allergies, ataxia telangiectasia, and sino respiratory infections.

Because they are prevalent in varying serum concentrations, a deficiency may be masked when only measuring total IgG. For this reason, the measurement of IgG subclass concentrations is important in the assessment of the immune system.

Associated diseases:

Immunoglobulin G subclass 4-related disease (IgG4-RD) is a recently recognized systemic inflammatory disease characterized by an elevated serum IgG4 level and an IgG4-positive lymphocyte infiltrate mainly in exocrine tissues.

Immunoglobulin subclass 4 (IgG4)-associated diseases are an increasingly recognized group of autoimmune diseases, which are characterized by sclerosis and lymphoplasmocytic infiltration of affected organs with IgG4-positive cells. 

The disease can affect the pancreas, biliary tract, salivary glands, retroperitoneum, lymph nodes, kidney, lungs or the prostate. In the gastrointestinal tract, autoimmune pancreatitis (AIP) and IgG4-associated cholangitis (IAC) are considered the most common manifestations.

References:

- https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3277054/

- https://pubmed.ncbi.nlm.nih.gov/29279461/

- https://journals.lww.com/ajg/abstract/2019/10001/741_low_serum_levels_of_igg4_antibodies_may.741.aspx