Immunoglobulins

Depending upon the character of the heavy chain, immunoglobulins are divided into five classes – IgG, IgD, IgE, IgA, IgM – that are expressed in different tissues.

Their frequency of occurrence is G > A > M > D > E.

The classes are further subdivided into isotypes, which have different properties in terms of complement fixation and binding to Fc receptors for immunoglobulins.

 Fc receptors  Immune Cytokines  Interferons

Ig class	tissue location/function
IgG	primary immunity against invading pathogens
IgA	mucus – gut, respiratory tract
IgM	early B cell-mediated response to invading pathogens
IgD	antigen receptor on B cells
IgE	mast cells – releases histamines in response to allergens
Ig class - heavy chains	humoral	cellular
IgG - γ - Gamma monomers	Fixes complement	Opsonin promotion of phagocytosis. Macrophages, monocytes, neutrophils, and some lymphocytes have Fc receptors for the Fc region of IgG. (not IgG2 or IgG4) Only Ig class that crosses placenta.
IgA - α - Alpha monomer or dimer (J chain)	Secretions - secretory piece or T piece attached (11S immunoglobulin). No complement fixation.	IgA can bind to neutrophils and some lymphocytes.
IgM - μ - Mu J chain in pentamer (19s); can be a monomer	Fixes complement	IgM binds to some cells via Fc receptors
IgD - δ - Delta monomer	No complement fixation	On B cell surfaces, IgD functions as an antigen receptor, and has extra amino acids at C-terminal end for anchoring to the membrane. IgD also associates with the Ig-α and Ig-β chains.
IgE - ε - Epsilon monomer with extra domain in the constant region	No complement fixation	Binds very tightly to Fc receptors on basophils and mast cells, so involved in allergic reactions

The immunoglobulin superfamily is evolutionarily ancient, is widely expressed, and is constitutive or long-term up-regulated. Immunoglobulin antibodies are released by activated B cells of the immune system, on which they also act as surface marker proteins. Adherence of immunoglobilins to foreign substances or to cellular invaders may be sufficient to disarm the invader, or the antibodies attached to foreign substances function as attack signals to macrophages and cytotoxic T cells.

Adhesion molecules of the immunoglobulin supergene family, activate specific kinases through phosphorylation, resulting in activation of transcription factors, increased cytokine production, increased cell membrane protein expression, production of reactive oxygen species, and cell proliferation.