Background The bursa of Fabricius provided the microenvironment for B‐cell differentiation. Continuous contact between lymphoid cells and antigen in the bursa further suggested that antigenic material has an important influence on the maintenance and development of B cells in the bursa. In addition, a dendritic cell, the bursal secretory dendritic cell (BSDC), has been identified in the medulla. The hypothesis that, in the bursal follicles, the contact between the lymphoid cells and the antigen may be mediated by dendritic cells prompted us to identify a bursal dendritic cell that becomes activated after contact with the antigen. Methods A polyclonal antiserum to S‐100 protein was used to identify bursal dendritic cells because S‐100 protein, a calcium‐binding protein, has been shown to be a marker for the identification of chicken dendritic cells following recent contact with antigen. Results At every age investigated, S‐100‐positive cells showed a location and shape identical to those described for BSDCs. Positive cells were found within and under the follicle‐associated epithelial cells (FAE), indicating that these cells were strategically placed where they would encounter the antigen. In addition, positive cells were found arranged along the corticomedullary junction, which is a regenerative zone for the BSDC. After 10 weeks of age, the number of positive cells dramatically decreased, suggesting that the endocytic activity of the FAE may become impaired as the bursa regresses. Conclusions The polyclonal antiserum to S‐100 protein identified the BSDCs in the bursal follicles. Positive cells may be BSDCs that have undergone a functional activation after contact with the antigen. These cells may have a role as antigen‐presenting cells in the bursal follicles. Hence, these cells may be involved in the events that lead to B‐cell differentiation. © 1996 Wiley‐Liss, Inc.

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In situ enzymatic generation of bimetallic nanoparticles, mainly Au/Pt, overcomes the drawbacks (continuous absorbance drift, modest LOQ, and long-time reaction) observed when AuNP alone are produced. In this study, Au/Pt nanoparticles have been characterized by EDS, XPS, and HRTEM images using the enzymatic determination of tyramine with tyramine oxidase (TAO) as a model. Under experimental conditions, the Au/Pt NPs show an absorption maximum at 580 nm which can be related to the concentration of tyramine in the range 1.0 × 10-6M to 2.5 × 10-4M with a RSD of 3.4% (n = 5, using 5 × 10-6M tyramine). The Au/Pt system enables low LOQ (1.0 × 10−6 M), high reduction of the absorbance drift, and a significant shortening of the reaction time (i.e., from 30 to 2 min for a [tyramine] = 1 × 10−4M); additionally, a better selectivity is also obtained. The method has been applied to tyramine determination in cured cheese and no significant differences were obtained compared to a reference method (HRP:TMB). The effect of Pt(II) seems to involve the previous reduction of Au(III) to Au(I) and NP generation from this oxidation state. Finally, a three-step (nucleation-growth-aggregation) kinetic model for the generation of NPs is proposed; this has enabled us to obtain a mathematical equation which explains the experimentally observed variation of the absorbance with time. Graphical abstract