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Anti-SARS-CoV-2 IgG

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Anti-SARS-CoV-2 IgG – Immunoglobin G (IgG) is the most common antibody in body, representing approximately 75% of serum antibodies in human blood. After an infection, it takes weeks for levels of IgG specific to the pathogen to increase in circulation, but levels remain elevated for months to years. The presence of IgG antibodies specific to a given pathogen (e.g., the virus that causes chicken pox, etc.) suggest that a person has previously been exposed to that pathogen. Anti-SARS-CoV-2 IgG refers to IgG that specifically binds to antigenic components of the SARS-CoV-2 virus that causes COVID-19 (namely S1 domain of the spike protein).

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Name: Anti-SARS-CoV-2 Immunoglobulin G (IgG) 
Category: Health and inflammation
Type of test: Blood

Immunoglobulin G (IgG) is the most abundant type of antibody found in the human body, representing roughly 75% of total serum antibodies. Structurally, IgG represents a tetrameric quaternary structure, with two sets of identical heavy and light chains forming a Y-like shape. Within each prong of the structure there is a variable antigen binding site, which is responsible for recognition of pathogen-related structures and linked with IgG’s role in the adaptive immune response. 

The purpose of the antigen binding site within the structure is to allow for maximum chemical diversity between copies of IgG while simultaneously conserving the portion that is recognized by the immune system. Upon recognition of the target antigen by the fragment antigen binding region (Fab) phagocytic immune cells are able to then recognize the constant (Fc) portion using the Fc receptor. Recognition through this receptor then usually leads to phagocytosis of the tagged residue. Within the IgG class there are four distinct subclasses, each of which are present at different concentrations and elicit varying degrees of both receptor affinity as well as complement system activation. Due to its small size and exclusively monomeric structure, IgG is able to diffuse through tissues and is found in most places throughout the body, including in the brain. 

IgG participates primarily in acquired, and not innate, immunity. Because of this, changes in concentrations of IgG do not necessarily reflect an acute response by the immune system in the same way that cytokine responses might. Once produced, pathogen-specific IgG circulates and is able to recognize structure distinct to that microorganism, allowing for a more rapid and effective activation of specialized secondary immune cells. IgG is known to be chronically elevated in long term infections such as HIV, as well as multiple scleroses and cases of multiple myeloma. Insufficient IgG responses are associated with increased risk for a wide variety of different infections. 

We offer assays for total IgG in saliva and SARS-CoV-2 specific IgG in serum / plasma samples. The salivary total IgG assay does not test for levels of this antibody specific to a given pathogen, but rather assesses overall levels of IgG. On the other hand, the SARS-CoV-2 specific serum / plasma IgG assay assesses (qualitatively) whether or not anti-SARS-CoV-2 IgG is present in a sample. In other words, this assay identifies whether or not an individual is likely to have mounted an immune response to the virus that causes COVID-19 during the weeks or months prior to collection of the sample. 

Patidar, K. A., Parwani, R. N., & Wanjari, S. P. (2011). Correlation of salivary and serum IgG, IgA levels with total protein in oral submucous fibrosis. Journal of oral science53, 97-102. https://pubmed.ncbi.nlm.nih.gov/21467820/

Sapkal, G., Shete-Aich, A., Jain, R., Yadav, P. D., Sarkale, P., Lakra, R., ... & Majumdar, T. (2020). Development of indigenous IgG ELISA for the detection of anti-SARS-CoV-2 IgG. The Indian journal of medical research151, 444. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7530443/

Unkeless, J. C., Scigliano, E., & Freedman, V. H. (1988). Structure and function of human and murine receptors for IgG. Annual review of immunology6, 251-281. https://pubmed.ncbi.nlm.nih.gov/2968084/

Vidarsson G, Dekkers G, & Rispens T.  (2014). IgG subclasses and allotypes: from structure to effector functions. Frontiers in Immunology, 5, e520. (link)

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