A Comprehensive Introduction to Immunoglobulin G
Immunoglobulin G (IgG) is the most prolific antibody isotype within circulation, representing roughly 75% of total serum immunoglobulins in humans. Synthesis via plasma B cells produces four main subclasses of IgG, each with a varying affinity profile and abundance in serum. Nonetheless, each of the subclasses provide essential functions during different stages of the body’s immune response, and are primarily focused on infiltrating their ability to bind foreign viruses, bacteria and fungi. Several immune mechanisms enable this process to occur.
These include: IgG-mediated immobilization and pathogen binding via agglutination; opsonization of IgG onto pathogen surfaces to enable pathogen recognition and phagocytic ingestion; IgG-induced binding of tumour cells and recruitment of complement component 1q to initiate the complement cascade pathway and membrane attack complex, enabling tumour lysis, and finally the ability of IgG to also bind and neutralise toxin secretion. A review article by Weiner et al during 2010 additionally discussed the capacity of IgG to maintain levels in serum, via their ability to bind neonatal Fc receptors and initiate signalling, via use of either immunoreceptor tyrosine-based activation motifs (ITAMS) or immunoreceptor tyrosine-based inhibitory motifs (ITIMS). These processes are summarised in Figure 1.
Figure 1: An Overview of IgG Structure and Function (Weiner et al, 2010)
Subsequent research has additionally determined that IgG antibodies maintain association with viruses following cell infection, and facilitate intracellular immune responses to inactivate virions within the cytosol. A cytosolic IgG receptor, known as tripartite motif-containing 21 (TRIM21), is located within immune cells and is able to efficiently recruit inbound antibody-bound viruses. It does this through its affiliated E3 ubiquitin ligase activity, enticing viruses towards a proteasome. This process enables phagocytic breakdown of virions, prior to the presentation of viral genes on immune cell surfaces. Accordingly, this demonstrates that the protective mechanism for IgG is not only confined to the cell membrane but carries on within the cell itself.
In terms of structure, IgG is a large antibody composed of two heavy and two light chains, weighing a total 150kDa. Within these chains, an Fc domain is comprised within the constant region, whereas a Fab domain is comprised within the variable region. Both of these binding regions provide important functions in enabling specificity against antigens. The Fc region comprises an exceedingly preserved N-glycosylation site, encompassing a number of attached core-fucosylated diantennary structured N-glycans. These additionally bear bisecting GlcNAc and α-2,6-linked sialic acid residues.
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