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IgE

Over 30 years age, IgE was discovered to be the antibody isotype mediating immediate hypersensitivity reactions in humans [1].

An IgE isoform differs by the replacement of the 2 C-terminal aminoacids by 8 amino acids including a C-terminal cysteine. This isoform is present in plasma as part of covalent complexes of approximately 300 kDa and 338 kDa [2].

References:
[1]Coombs R. R. et al. Lancet 1, 1115-1118 (1968).
[2]Chan L. A. et al. Mol. Immunol. 37, 241-252 (2000).