Immunoglobulin IgG Overview

• Immunoglobulin isotopes are categorized into five classes: IgM, IgD, IgG, IgE, and IgA, distinguished by the constant region of the antibody.

IgG

• IgG has a molecular weight of 150 kDa.
• It predominates in blood, lymph, cerebrospinal fluid, and peritoneal fluid.
• Constitutes 15% of total serum proteins.
• Circulates as a monomer.
• Has a half-life of approximately 23 days.
• Has subclasses IgG1, IgG2, IgG3, and IgG4, differing in heavy chain constant region length.
• Deficiencies in IgG subclasses indicate specific conditions; IgG4 deficiency suggests allergy.
• The IgG2 subclass further divides into IgG2a (inflammation promotion) and IgG2b (antibody production promotion).
• IgG performs opsonization, coating pathogens for easy destruction.
• The Fab portion binds to the antigen of a bacteria.
• The Fc portion of IgG is detectable by phagocytes, such as macrophages, facilitating engulfment of the bacteria.
• IgG performs agglutination, forming precipitates to facilitate phagocytosis.
• IgG binds to bacteria, forming clusters to facilitate detection and destruction.
• IgG is the only immunoglobulin to cross the placenta (excluding IgG2).
• IgG receptors on the placenta facilitate the transfer of immunity from mother to fetus, starting around 3-4 months of pregnancy.
• Performs antibody-dependent cell-mediated cytotoxicity, where the Fab region binds to abnormal cells like tumor cells, attracting natural killer cells via the Fc region to release cytotoxins and kill the tumor.
• It neutralizes toxins like tetanus, botulism, and snake/scorpion venom by attaching to and inactivating the toxins' active sites.
• IgG neutralizes viruses by binding to viral epitopes (antigens), preventing viral absorption and release.
• IgG activates the classical pathway of complement, stimulating an immune response.

IgM

• IgM has a molecular weight of 900 kDa.
• Forms a pentameric structure in the bloodstream when secreted, held together by disulfide bonds called the J chain.
• The first antibody produced and expressed upon exposure to an infectious agent.
• Possesses a short half-life of about 5 days.
• The pentameric structure makes it effective at activating complement and agglutination.
• The classical pathway of complement is activated by IgM.
• IgM has poor toxin and viral neutralization capabilities.
• Does not cross the placenta; IgG is the only antibody to do so.
• First immunoglobulin made by the fetus at five months.
• Exhibits isohemagglutinin activity.

IgD

• Molecular weight of 180 kDa.
• Not a secreted immunoglobulin; it is membrane-bound.
• Expressed on the membranes of B cells and plasma cells.
• Found in low concentrations in the serum (bloodstream).
• Can be co-expressed with IgM on B cells and is a marker for B-cell maturity.
• B cells expressing both IgM and IgD are not yet plasma cells.
• Full function of IgD is still unknown.

IgE

• Molecular weight of about 200 kDa.
• Secreted as a monomer.
• Has a half-life of about 2 days, which can increase when bound to mast cells or basophils.
• Does not agglutinate or fix complement.
• Binds to mast cells and basophils, increasing its half-life to about two weeks.
• Mast cells have Fc epsilon receptors for IgE antibodies.
• When IgE bound to mast cells binds to antigens of pathogens, it triggers the mast cell to release granules containing histamine.
• Histamine is an inflammatory mediator implicated in hypersensitivity reactions.
• Mediates allergic reactions via histamine release from mast cells.
• Has some function in protection against parasites, such as worms.

IgA

• Has a molecular weight of about 165 kDa.
• Can be secreted as a monomer or dimer, especially in the mucosal system, found in the respiratory tract, genital tract, and gut.
• In dimeric form, IgA is held together by a J chain.
• Has a half-life of about 5.5 days.
• It Serves as the main immunoglobulin in mucus, providing primary defense against mucosal infections.
• There are two subclasses of IgA: IgA1 (93%) and IgA2 (7%).
• Defends against mucosal infections such as HIV, Helicobacter pylori, and Streptococcus pneumoniae.
• Does not fix or activate complement to avoid inflammatory responses in the mucosal tract.
• Exhibits agglutination and antiviral capacity, crucial for defense in the mucosal system.
• The primary antibody found in the mucus.

Overview of Immunoglobulin G (IgG), including its structure, subclasses (IgG1, IgG2, IgG3, IgG4) and functions. IgG is the most abundant antibody isotype in human serum. The antibody molecule mediates opsonization and agglutination.