IgG is a monomeric immunoglobulin, built of two heavy chains ?? and two light chains. Each molecule has two antigen binding sites. This is the most abundant immunoglobulin and is approximately equally distributed in blood and in tissue liquids. It is important that this is the only isotype that can traverse trough the placenta, because this gives protection to the fetus in its first weeks of life, when its own immune system has not yet developed. It can bind to many kinds of pathogens, for example viruses, bacteria, and fungi and protects the body against them by complement activation (classic pathway), opsonization for phagocytosis and neutralisation (immunology) of their toxins. There are 4 subclasses: IgG1, IgG2, IgG3, and IgG4.
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