During a transfusion, if a patient develops an anaphylactic, nonhemolytic reaction, which transfused substance most likely caused it?

An anaphylactic, nonhemolytic reaction during a transfusion is most commonly associated with the presence of immunoglobulin A (IgA). This type of reaction can occur in individuals who are IgA-deficient and have developed antibodies against IgA. When transfused with blood products containing IgA, these patients can experience a severe allergic reaction, leading to anaphylaxis.

The likelihood of developing this reaction is particularly relevant in the context of blood transfusion because many donors may have a natural concentration of IgA in their plasma. If a patient has IgA antibodies and receives such blood, they can mount a rapid immune response leading to anaphylaxis.

Other substances listed, like the Group A antigen or Rho (D) antigen, are related to blood group antigens and can cause hemolytic reactions if incompatibility occurs but are not typically associated with anaphylaxis. Likewise, antigens in the Duffy system do not play a significant role in triggering anaphylaxis. Thus, IgA is the substance most directly linked to the anaphylactic reaction in this context.