Immunoglubulin G (IgG) is a glycoprotein molecule which belongs to the immunoglobulin family of proteins known as antibodies. Immunoglobulins are the key component of humoral immunity. IgG is a monomeric immunoglobulin, built of two heavy chains gamma and two light chains. The heavy chains are linked to each other and to the light chain by disulfide bonds. Each molecule has two antigen binding sites. While the amino-terminal portions that exhibit highly variable amino-acid composition are involved in antigen binding, the C terminal constant parts are involved in complement binding, placental passage and binding to cell membrane.
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