What antibody is typically involved in a type II transfusion reaction for a patient with blood group O transfused with group A blood?

In a type II transfusion reaction, the primary antibodies involved are typically IgM antibodies. When a patient with blood group O receives blood from a group A donor, the O blood type has natural antibodies against A antigens, primarily of the IgM class. These antibodies can bind to the A antigens present on the transfused red blood cells, leading to the activation of the complement system and subsequent hemolysis of the transfused cells.

IgG antibodies also play a role in certain hemolytic reactions, but in the context of transfusion reactions and the specific scenario presented, it is the IgM antibodies that are primarily responsible for the immediate response due to their pentameric structure, allowing strong binding to antigens and effective complement activation.

IgE is associated more with allergic reactions and not typically involved in blood group incompatibility reactions. Additionally, while IgG can be involved in delayed reactions or sensitization processes, the prompt response in this case is attributed to IgM as the main antibody type in a type II hypersensitivity scenario involving ABO incompatibility. Therefore, the role of IgM in this specific transfusion reaction is critical to understanding the mechanism behind the severe and rapid immune response.