抗体アイソタイプ：IgG

Immunoglobulins, also known as antibodies, are heterodimeric proteins made up of two heavy (H) chains and two light (L) chains. They can be classified into five primary isotypes (IgG, IgA, IgD, IgE, and IgM) determined by the constant domains in their H chains, which give each isotype their different characteristics.

Immunoglobulin G is the most abundant subtype, being present in the largest concentrations at 75-80% of IgGs in blood and tissue fluids. IgG makes direct impacts on the immune response with high neutralization and opsonization activities towards toxins and viruses, acting as the first line of defense after memory B cells trigger a response. They are responsible for both existing and acquired immunity against previously encountered antigens. Once IgG binds to antigens, the Fc receptor can bind to macrophages, which can carry out phagocytosis on the intruder.^1,2

Uniquely, IgG can also cross the placenta and impart immunity to the fetus. This occurs through transcytosis, in which the IgG is endocytosed and shuttled across the cytoplasm to be secreted towards the basal cell membrane of syncytiotrophoblasts. Once there, the complex is released into fetal circulation.

IgG can be further classified into: IgG1, IgG2, IgG3, and IgG4. All four subtypes exhibit different functional activities. For example, with a HIV patient, IgG3 antibodies have been demonstrated to be more effective at neutralizing HIV over IgG1, likely because of differing antibody flexibility affecting access and how changes are induced in the virus’ oligomer structure.³

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References:

1. Schroeder, H. W., & Cavacini, L. (2010). Structure and Function of Immunoglobulins. The Journal of Allergy and Clinical Immunology, 125(2 0 2), S41. https://doi.org/10.1016/J.JACI.2009.09.046 

2. Borad, S. G., & Singh, A. K. (2022). Immunoglobulins. Encyclopedia of Dairy Sciences, 894–900. https://doi.org/10.1016/B978-0-12-818766-1.00367-6 

3. Ciobanu, A. M., Dumitru, A. E., Gica, N., Botezatu, R., Peltecu, G., & Panaitescu, A. M. (2020). Benefits and Risks of IgG Transplacental Transfer. Diagnostics, 10(8). https://doi.org/10.3390/DIAGNOSTICS10080583