Institute of Virology, Vaccines and Sera - Torlak, Vojvode Stepe 458, 11221 Belgrade, Serbia.
Med Hypotheses. 2011 Dec;77(6):959-61. doi: 10.1016/j.mehy.2011.08.018. Epub 2011 Sep 7.
Immunoglobulin M (IgM) is the first antibody produced upon infection, and is often suggested as the first line of defense of human immune system. In addition to being present on the surface of naïve B cells as a monomeric molecule, IgM is always secreted as a polymer. The most abundant IgM polymer in humans is pentamer, composed of five monomeric units, joined together by so-called joining or J chain. On the other hand, it is well known that hexameric IgM can be also found in human sera. Its presence is often related to different dissorders (Waldenström's macroglobulinemia, cold agglutinin, and recurrent urinary bacterial infections), although it is believed that small amounts of hexamer are present in normal human sera as well. Unlike pentamer, IgM hexamer contains six monomeric blocks and completely lacks J chain. Although it has been decades since its discovery, the precise function of IgM hexamer is still unknown. Since it was documented that hexamer is very potent in activating complement, it is suggested that its production in humans must be under strict control, and that it is produced in special conditions, when strong activation of complement is absolutely needed. However, the question is whether hexameric IgM is really a secret weapon or just an undesirable molecule in humans. According to structural and known functional characteristics of both pentamers and hexamers of IgM, it can be concluded that hexamers are, in addition to being maybe too reactive to be around, probably not that efficient in protecting us from bacterial and viral infections.
免疫球蛋白 M（IgM）是感染后产生的第一种抗体，通常被认为是人体免疫系统的第一道防线。除了作为单体分子存在于幼稚 B 细胞表面外，IgM 通常以聚合物的形式分泌。人类中最丰富的 IgM 聚合物是五聚体，由五个单体单元组成，通过所谓的连接或 J 链连接在一起。另一方面，人们已经知道六聚体 IgM 也可以在人血清中发现。它的存在通常与不同的疾病有关（瓦尔登斯特伦巨球蛋白血症、冷凝集素和复发性尿路感染），尽管人们认为正常人体血清中也存在少量六聚体。与五聚体不同，IgM 六聚体包含六个单体块，完全缺乏 J 链。尽管自发现以来已经过去了几十年，但 IgM 六聚体的确切功能仍然未知。由于已经证明六聚体在激活补体方面非常有效，因此人们认为它在人类中的产生必须受到严格控制，并且只有在需要强烈激活补体的特殊条件下才会产生。然而，问题是六聚体 IgM 是否真的是人类的秘密武器，还是只是一种不受欢迎的分子。根据 IgM 五聚体和六聚体的结构和已知功能特征，可以得出结论，除了可能过于活跃而无法存在之外，六聚体在保护我们免受细菌和病毒感染方面可能效率不高。
