Specificity and cross-reactivity of antibodies
Specificity is a commonly discussed concept in the context of antibodies. It can have different meanings. Sometimes, it is used simply to indicate that the antibody has high affinity for anti-gen. Generally this means that the antibody has a combining site that fits very well to an epitope on the antigen and is much less likely to fit other shapes very well. Therefore it is specific for the antigen. However, there may be other shapes that can be accommodated, especially if they are related to the antigenic epitope in composition or character. Most likely is that other molecules will be recognized with lower affinity. It is important to remember from the discussion above that antibodies will be functional at concentrations around their Kd values. So if an antibody has a nM affinity for a given antigen and is present at nM concentrations in vivo, cross‐reactivities with other antigens in the sub‐μM range are unlikely to be functionally significant unless those antigens are at high concentrations.
A second meaning that is attached to “specificity” is the ability to discriminate between molecules. This clearly overlaps with the discussion above but could also be applied to lower affinity antibodies. Thus in genomic studies there has been a demand for antibodies that can distinguish target proteins from many other proteins and identify the target proteins in a variety of assays. This has not necessarily required high affinity but has required good discrimination. Moderate‐affinity antibodies selected from phage libraries have been used successfully in this arena.