Which immunoglobulin may form insoluble aggregates upon thawing, giving a false-negative result with previously frozen serum samples?

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Blood for viral serology testing should be transported to the laboratory in the sterile tube in which it was collected. Serum should be separated from the clot as soon as possible. Serum can be stored for hours or days at 4° C or for weeks or months at 20° C or below before testing. Testing for virus-specific IgM should be done before freezing whenever possible, because IgM may form insoluble aggregates upon thawing, giving a False-negative result.

Which Immunoglobulin May Form Insoluble Aggregates Upon Thawing, Giving a False-Negative