Mast-cell degranulation is triggered by the bridging of Fc receptor-bound antigen-specific immunoglobulin IgE on the cell surface. In vitro experiments suggest that antibody affinity and nonspecific IgE may affect the mast-cell function, however, their importance in vivo is unclear. Investigations of the effects of these parameters on mast-cell sensitization were therefore carried out in a rat immunization model in which the IgE response is transient and peaks on days 10–15. Between these two timepoints, significant changes in the level of specific IgE were not observed, but the avidity of specific IgE increased (P < 0.05). Total serum IgE peaked on day 10 and slowly declined, with the relative proportion of specific to total IgE increasing from day 10–15 (P < 0.05). Despite similar levels of antigen-specific IgE, increasing avidity and an increased proportion of specific IgE between days 10 and 15, the biological activity of IgE in the serum peaks on day 10 and declines rapidly, dropping around seven-fold by day 15 (P < 0.001). Mechanisms that could explain this finding, such as differential expression of IgE isoforms and changes in the fine specificity of the IgE response, are discussed.