Based on the results 1:1, 1:2, 1:4, 1:8, 1:16, 1:32, 1:64, what is the antibody titer?

The antibody titer is determined by identifying the highest dilution of serum that still demonstrates a positive reaction, indicating the presence of antibodies against the target antigen. In the provided results (1:1, 1:2, 1:4, 1:8, 1:16, 1:32, 1:64), one must examine the dilution at which the last positive reaction occurs.

Starting from the 1:1 dilution, each subsequent dilution represents a decreasing concentration of the antibody. The positive reactions continue through the 1:2, 1:4, and 1:8 dilutions, indicating the presence of antibodies at these concentrations. The positive reaction stops at the 1:16 dilution, meaning that 1:32 and 1:64 demonstrate no evidence of antibody presence.

Consequently, the highest dilution with a positive reaction is 1:16, which is identified as the antibody titer. Therefore, the correct answer indicates that the titer, represented as a whole number, is 16.