For many years, biologists have been interested in toxicology to assess the effects of contamination on humans. In recent years, researchers have found a new evaluation technique based on the analysis of dendritic cells in vitro. Up to now the analysis conducted on these cells remains purely visual in nature. Therefore, it is subjective and time-consuming because of the different morphological features of the cells. Here, we suggest to use automatic processing techniques by image analysis. The foremost goal of this paper is to suggest an assessment tool for the analysis of immunotoxic effects of food contaminants (mycotoxins) on the immune system using automatic segmentation techniques of microscopic images of dendritic cells. The suggested method is based on automatic thresholding and mathematical morphology. For validation purposes, an experimental study is carried out on 55 microscopic images of dendritic cells visually analyzed by an expert in order to make comparisons or to have a reference segmentation of the cells. Results show that the proposed approach is efficient in identifying dendritic cells with a segmentation accuracy of 95%.