Which statement is incorrect regarding immune antibodies?

• A. Most often found in the MNSs system
• B. Mainly IgG
• C. Can cross the placenta
• D. React best with antiglobulin

Answer: (A)

Immune antibodies are alloantibodies formed after exposure to foreign antigens, and they are typically of the IgG class. Because IgG can cross the placenta, these antibodies often have clinical significance in transfusion and can cause hemolytic reactions or fetal-metal transfusion issues. They also react best in the antiglobulin (Coombs) phase, which is why the indirect/direct Coombs tests are used to detect them.

The statement that immune antibodies are most often found in the MNSs system is not accurate. While antibodies against M or N can occur, many clinically significant immune antibodies target other systems—most notably Rh, Kell, Kidd, and Duffy. The MNSs antibodies are frequently IgM and cold-reactive, hence not representative of the typical immune antibodies encountered in transfusion practice. The other statements align with the usual properties of immune antibodies: they are mainly IgG, they can cross the placenta, and they react best with antiglobulin.