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immunoglobulins

introduction

  • a group of “antibody” proteins which form the “humoral” immune system and are produced by B lymphocytes

IgA

  • a type of immunoglobulin that protects against infections of the mucous membranes lining the mouth, airways, and digestive tract
  • serum IgA levels may be raised due to:
    • pulmonary and gastrointestinal inflammatory diseases
    • some autoimmune conditions
    • liver disease
    • plasma cell disorders

IgE

  • immunoglobulin generally associated with allergic reactions, and parasitic infections

IgG

  • composed of two identical heavy (CH) chains and two identical light (L) chains
  • usually have 2 identical antigen binding sites (on the Fab arms)
  • the amino acid sequences of the various IgG subclasses are 90 to 95 percent homologous
  • CH regions are encoded by C gene segments clustered on chromosome 14
  • four subclasses: IgG1, IgG2, IgG3, and IgG4
  • functional differences among the subclasses result from structural variation in both the Fc regions and the hinge regions
  • IgG1 and IgG3 fix C1q most effectively, IgG2 weakly, and IgG4 not at all
  • the Fc region is important in binding to phagocyte membrane Fc receptors and in initiating phagocytosis following “opsonization” of the target antigen by it being bound to IgG
  • 60-70% of serum IgG is IgG1, 20-30% is IgG2, 5-8% is IgG3 and 1-4% is IgG4
  • half-lives are ~ 21 days although IgG3 half-life is only ~9 days
  • serum IgG levels may be raised due to:
    • chronic active infection or inflammation
    • in association with plasma cell disorders

IgM

  • acute phase antibody response to an antigen
  • raised serum IgM levels may be due to:
    • nonspecific marker of inflammation
    • can be associated with liver disease

immunoglobulin production diseases or gammopathies

excess production

  • benign monoclonal IgG paraproteinaemia
  • hyper IgM syndrome
    • family of 5 genetic disorders characterised by relatively high IgM levels
    • causes increased risk of infections and usually have thrombocytopenia and neutropenia
  • Wiskott-Aldrich syndrome
    • rare X-linked recessive disease characterized by eczema, thrombocytopenia, immune deficiency, and bloody diarrhoea
    • IgM levels are reduced, IgA and IgE are elevated, and IgG levels can be normal, reduced, or elevated
  • hyper-IgE syndrome
    • causes eosinophilia, high levels of IgE, retained primary teeth, recurrent “cold” staphylococcal infections, recurrent candidial infections, unusual eczema-like skin rashes, severe lung infections that result in pneumatoceles
    • AD and AR forms; thought to be caused by abnormal neutrophil chemotaxis due to decreased production of interferon gamma by T lymphocytes
  • raised IgG4 levels:
    • IgG4-related systemic disease (IgG4-RSD)
      • rare
      • autoimmune pancreatitis
      • sialadenitis and salivary gland enlargement
      • tumor-like swelling of involved organs
    • may also occur in:

deficiency or dysgammaglobulinemia

  • X-linked agammaglobulinemia
    • complete lack of circulating B cells; 1 in 100,000 male newborns; Dx usually after a history of recurrent infections, mostly in the respiratory tract, through childhood
  • transient hypogammaglobulinemia of infancy
  • IgA deficiency
    • mild illness, mostly asymptomatic but increased risk of infections such as sinusitis, chest infections and UTI
    • risk of anaphylaxis to blood transfusions or intravenous immunoglobulin due to the presence of IgA in these blood products
    • higher risk of developing autoimmune diseases in middle age
    • 1 in 300 people;
  • IgG sub-class deficiencies
  • isolated primary IgM deficiency
  • common variable immunodeficiency
  • immunodeficiency–centromeric instability–facial anomalies syndrome (ICF) syndrome
    • very rare AR disorder; childhood infections; hypertelorism, low-set ears, epicanthal folds and macroglossia.
ig.txt · Last modified: 2020/08/18 17:37 by gary1