immunoglobulin Syllabification: (im·mu·no·glob·u·lin)Pronunciation: /ˌimyənōˈgläbyələn, iˌmyo͞o-/Definition of immunoglobulin noun Biochemistry any of a class of proteins present in the serum and cells of the immune system, that function as antibodies. immunoglobulin in other Oxford dictionaries Definition of immunoglobulin in the British & World English dictionary