Human immunoglobulin G (IgG) includes four different subtypes (IgG1, IgG2, IgG3, and IgG4), and it is also now appreciated that there are genetic variations within IgG subtypes (called isoallotypes). Twenty‐nine different isoallotypes have been described, with 7, 4, 15, and 3 isoallotypes described for IgG1, IgG2, IgG3, and IgG4, respectively. The reactivity of anti‐IgG with different isoallotypes has not been characterized.

A novel monoclonal anti‐K antibody (PugetSound Monoclonal Antibody 1 [PUMA1]) was isolated and sequenced, and a panel of PUMA1 variants was expressed, consisting of the 29 known IgG isoallotypes. The resulting panel of antibodies was preincubated with K‐positive red blood cells (RBCs) and then subjected to testing with currently approved anti‐IgG by flow cytometry, solid phase systems, gel cards, and tube testing.

A US Food and Drug Administration (FDA)‐approved monoclonal anti‐IgG (gamma‐clone) failed to recognize 2 of 15 IgG3 isoallotypes (IgG3‐03 and IgG3‐13) and 3 of 3 IgG4 isoallotypes (IgG4‐01, IgG4‐02, and IgG4‐03). In contrast, an FDA‐approved rabbit polyclonal anti‐IgG recognized each of the known human IgG isoallotypes.

These findings demonstrate “blind spots” in isoalloantibody detection by a monoclonal anti‐IgG. If a patient has anti‐RBC antibodies predominantly of an IgG3 subtype (the IgG3‐03 and/or IgG3‐13 variety), then it is possible that a clinically significant alloantibody would be missed. IgG‐03 and IgG‐13 have an estimated frequency of 1% to 3% in Caucasian populations and 20% to 30% in certain African populations. Nonreactivity with IgG4 is a known characteristic of this monoclonal anti‐IgG, but IgG4 isoallotypes have not been previously reported.