Biphasic Behavior of Anti-M Antibody and Hemolytic Disease of the Fetus and Newborn (HDFN).

The anti-M antibody is a cold, naturally occurring immunoglobulin M (IgM) antibody that is generally considered clinically insignificant and often overlooked in transfusion practices and assessments of patients at risk for hemolytic disease of the fetus and newborn (HDFN). However, the presence of an IgG component in this case renders the antibody clinically significant, underscoring the necessity for proper serologic testing during prenatal evaluations. We present a case involving an anti-M antibody with an IgG component to highlight the critical importance of thorough serologic testing during prenatal testing.