Which immunoglobulin class is produced by transfusion or pregnancy?

• A. IgM
• B. IgG
• C. IgA
• D. IgE

Answer: (B)

When foreign red cell antigens are encountered through transfusion or during pregnancy, the antibody response that develops is predominantly IgG. IgG is small enough to cross the placenta, so maternal IgG antibodies against fetal red cells can reach the fetus and cause hemolysis, which is the basis of hemolytic disease of the fetus and newborn. In transfusion medicine, clinically significant alloantibodies that can cause transfusion reactions are also mainly IgG, because this class can opsonize donor red cells and promote their clearance. Other classes—IgM (produced early but does not cross the placenta) and IgA or IgE (having different roles)—are not the primary antibodies involved in this transfusion/pregnancy context. Therefore, IgG best fits the scenario.