We previously showed that IgH sequence alone minimally influenced germinal centre (GC) B‐cell survival fate. As end‐stage effector B cells are typically more mutated than founder GC B cells, we worked to develop an assay that would enrich for populations of GC B cells with progressively increasing numbers of somatic mutations, which could potentially be used as an indicator of positive selection. We targeted CD45 as it has been shown to influence activation‐induced cytidine deaminase (AID) expression. In this study, anti‐CD77 and anti‐CD45RO (RO) were used to subdivide CD19+IgD−CD38+CD77+ centroblasts (CB) and CD19+IgD−CD38+CD77− centrocytes (CC) into three contiguous RO fractions (RO−, RO+/− and RO+) and assessed whether mutation frequency and characteristics associated with selection varied with respect to increasing RO expression. Here, we show that the average number of mutations per IgVH4 transcript increased concordantly with RO for CC, but not for CB. CC also exhibited an RO‐associated increase in replacement mutations. Comparative analysis of clonally related sequences revealed that increased mutations were not due to the exclusive persistence of surface RO on highly mutated cells. RO‐expressing CC and CB pools showed increased signs of activation (CD69+) and were enriched for surface Ig+ cells. BCR‐crosslinking induced a significant increase in surface RO on total tonsillar and GC B cells, which collectively suggests that the RO‐associated increase in mutations is attributable, at least in part, to the cycling of cells that may have recently undergone BCR‐mediated selection, or are potentially in developmental transition between CC and CB stages.

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In order to study the nature of receptors for immune complexes (IC) on the surface of platelets, the release of radiolabelled serotonin produced by exposure of platelets to IC of known composition was studied. Free plasma components might alter this reaction, so platelets were used both in platelet rich plasma (PRP) and after gel filtration. As it has been thought that the aggregation of the immunoglobulin molecules through the antigen was neccessary for the release reaction, we have studied complexes made with antigens of different valencies. The antigen was albumen substituted with different numbers of dinitrophenol (DNP) groups, and the antibody rabbit-anti DNP. Polyvalent antigen complexes were also made from bovine serum albumen (BSA) and rabbit anti-BSA antibody. Soluble and insoluble IC made in a variety of ratios with polyvalent antigen induced release both in PRP and in gel filtered platelets. Soluble complexes in antigen excess were the most effective. Complexes of monovalent antigen prepared, using two methods of monovalent substitution, were also capable of inducing release in PRP and in gel filtered platelets, although the complex could not exist in an aggregated form. Release was less than that produced by polyvalent complex, but could be increased by the addition of CI.These results show that, at least with monovalent IC, the simultaneous involvement of more than one receptor is not an absolute requirement for platelet release. These receptors for monovalent complexes could be different from the Fc receptors for aggregated immunoglobulins or polyvalent complexes.