An analysis of B cell selection mechanisms in germinal centers
  Affinity maturation of antibodies during immune responses is achieved by multiple rounds of somatic hypermutation and subsequent preferential selection of those B cells that express B cell receptors with improved binding characteristics for the antigen. The mechanism underlying B cell selection has not yet been defined. By employing an agent-based model, we show that for physiologically reasonable parameter values affinity maturation can neither be driven by competition for binding sites nor antigen -- even in the presence of competing secreted antibodies. Within the tested mechanisms, only clonal competition for T cell help or a refractory time for the interaction of centrocytes with follicular dendritic cells are found to enable affinity maturation while generating the experimentally observed germinal center characteristics and tolerating large variations in the initial antigen density. 
Michael Meyer-Hermann, Philip K. Maini, Dagmar Iber