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Wikipedia says that NCAM (CD56) glycoprotein belongs to Immunoglobulin (Ig) superfamily.

A the same time, its article on antibodies equates them with immunoglobulins. NCAM is obviously not an antibody, so are these words really synonymous?

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See also https://en.wikipedia.org/wiki/Immunoglobulin_superfamily

What you have here is a bit of imprecise terminology, and a language problem equivalent to another familiar one: cats. A lion is both a cat and not a cat, because 'cat' refers both to a family of mammals and to what is more precisely referred to as a 'house cat'. To tell which you are talking about requires context or clarification.

Same thing here: antibodies are referred to as immunoglobulins, which are part of a larger family, the immunoglobulin superfamily, which you might also refer to as immunoglobulins but usually not. They share a name because the immunoglobulins (antibodies) are sort of the 'archetype' of the superfamily. Wikipedia is saying CD56 belongs to the immunoglobulin superfamily - that's clear because they specifically write superfamily - not quite the same as calling them an "immunoglobulin". They're also saying antibodies are called immunoglobulins, but that doesn't mean everything in the immunoglobulin superfamily is an antibody.

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    $\begingroup$ I don’t think your cat analogy is so helpful because we can say “a lion is a cat” but we can’t say “NCAM is an antibody”. I would have made the point that immunoglobulin is the technical name for a molecule of precise function, commonly known as an antibody. However “immunoglobulin superfamily” is a term used in structural classification of proteins which have a variety of different functions. It could equally well have been named something like the Y family, but was named after the first structure of this type to be determined. $\endgroup$
    – David
    Commented Dec 22, 2022 at 19:14
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    $\begingroup$ @David Eh, it would be better to argue that "we can't say 'NCAM is an immunoglobulin' " - I think that's accurate, in that "immunoglobulin" is only ever used for the antibody, and not in a broader sense to refer to the whole cat family/immunoglobulin superfamily, yet it's still used in that broader sense in the adjective form, e.g. immunoglobulin domain. The point of my answer is that you need to distinguish between the immunoglobulin superfamily and immunoglobulins/antibodies, just like you need to distinguish between the cat family and the household cat. $\endgroup$
    – Bryan Krause
    Commented Dec 22, 2022 at 23:24
  • $\begingroup$ Whatever. But if you must make an analogy the it should be: Cat is the common name for Felis catus, a member of the family felidae. In the same way Antibody is the common name for immunoglobulin G (generally) a member of the immunoglobulin structural superfamily. But then it doesn't work as nicely. I prefer to stick to pictures of proteins, as in your Wikipedia link. May get round to it myself over Christmas. $\endgroup$
    – David
    Commented Dec 23, 2022 at 11:57
  • $\begingroup$ @David I'll be glad to see your answer. $\endgroup$ Commented Dec 24, 2022 at 16:18

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