In double immunodiffusion reactions, the precipitin band is:

In double immunodiffusion reactions, the precipitin band forms at the point of equivalence between the antigen and antibody. The correct understanding of the precipitin band is that it is indeed concave toward the protein of greatest molecular weight. This occurs due to the differences in mobility of the proteins involved; larger proteins diffuse more slowly compared to smaller proteins. Consequently, as the antigen and antibody diffuse toward each other, the precipitin band appears where the concentrations of antigen and antibody are optimal for binding. The curvature of the band reflects the characteristics of the proteins involved, indicating that the larger, heavier proteins will create a concave shape in relation to their position in the gel.

This insight highlights the dynamic interactions at play between the antigens and antibodies, helping to visualize and understand how differential sizes influence the band formation in double immunodiffusion.