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.
