Affinity maturation (AM) in a germinal center (GC).

Simulations of the GC reaction starting with 20 different B cell clones. During AM, cells mutate their BCR upon division, changing only the BCR interaction energy with the Ag (EAg−Ab) and thus the off-rate r0 of the BCR from the Ag. (a) The mean affinity (ω = q/r0) of the dominant clone at day 16 of the GC reaction in the competitive phase, as a function of spike density nAg. (b) Mean number of Ag molecules captured by the dominant clone at day 16 as a function of the spike density. (c) The capture probability of Ag molecules by a single BCR (Eq (S10)) when the binding energy of the BCR is EAg−Ab = 1.5. The other parameters of the captures process are listed in Table 2. (d) The probability of selecting the weaker affinity B cell (see text) during the competitive phase. This probability depends on the spike density (Eq (S11)). We sampled energies of B cell pairs from the population energy distribution (S5 Fig) obtained in the simulation results shown in (a), to estimate the probability of the lower affinity cell proliferating before a higher affinity one, at different times during the GCR. The relative mild variation in this figure are amplified in multiple rounds of selection.