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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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.