Figure 1

Coevolution from the population to the molecular scale. Recognition starts at the molecular level through the specific binding of antibodies to bits of the viral proteins called epitopes, illustrated on the right by the co-crystal structure of an antibody fragment and the hemagglutinin protein of influenza (from [1]). At the other end of the population scale (on the left), effective descriptions of antibodies and viruses in a common antigenic space explain how viruses (in purple) are pushed to mutate continually under the pressure of the immune system of its target hosts (in blue), growing faster in regions of the antigenic space where the population is susceptible. At the intermediate scale of the organism (middle), the repertoire of antibodies, represented here by a list of sequences of their third heavy-chain Complementarity Determining Loop, evolves under the selective pressure of successive acute infections (or of a chronic infection) to optimize protection and to prepare the organism for future challenges. Only a small fraction of the whole repertoire (shown in blue) is specific to the viral epitope, and is structured into distinct lineages.