IgG (summary and causes of increased and reduced levels)

IgG is the main circulating immunoglobulin, and it is present both in plasma and in interstitial fluid. It has a monomeric structure and consists of four sub-classes; IgG1, IgG2, IgG3 and IgG4.

Selective deficiency of any of the IgG subclasses may occur but are easily missed as the dominating effect of IgG1 may result in normal levels of total IgG.

A working knowledge of the age of appearance of the subclasses is important in understanding the immune responses of the very young.

Note that IgA deficiency may be associated with IgG2 and IgG4 deficiency.

IgG is the only immunoglobulin which crosses the placenta in significant volume. Thus, any disease mediated by IgG may be transmitted, temporarily at least, to the foetus; the classic example of this is Grave's disease.

In brief (in adults):

• LOW IgG
  • IgG 5- 5.9g/L
    • mild hypogammaglobulinaemia is commonly nonspecific. Suggest repeat in 3 months. Suggest discussing with clinical immunology if there are recurrent bacterial infections
  • IgG 3-4.9g/L
    • moderate hypogammaglobulinaemia is often associated with recurrent bacterial infections. If persistently low, consider discussing with Consultant Immunologist
  • IgG 0-2.9g/L
    • significant hypogammaglobulinaemia confers a high risk of serious bacterial infections. Suggest check full blood count and consider secondary causes. Please contact Consultant Immunologist
      
      
• High IgG (>16g/L)
  • elevated IgG levels can be seen in chronic active infection or inflammation, or in association with plasma cell disorders.

In detail:

Causes of a raised IgG include:

• IgG myeloma
• chronic liver disease e.g. cirrhosis
• sarcoidosis
• chronic infection
• autoimmune disease
• parasitic diseases

Causes of a reduced IgG include:

• acquired immunodeficiency
• hereditary immunodeficiency
• pregnancy
• protein-losing syndromes
• Waldenstrom's macroglubulinaemia
• non-IgG myeloma

Reference:

• NHS Exeter Clinical Laboratory. Immunoglobulins (IGA, IGG, IGM) (Accessed 13/6/2019)