For many years, a number of mathematics didacticians have been studying the role of social interactions in the development of students' mathematical thinking. This text deals with this aspect of social interactions in the specific framework of how students with serious learning difficulties learn mathematics in the special education classroom. The point of view adopted is inspired by the work of Vygotski (1994). The ideas of this author, particularly his ideas on defectology, highlight several problems that are still very pertinent today in the development and education of students with learning difficulties. This text first presents Vygotski's fundamental ideas. The results of a study conducted specifically in a special class with students with serious learning difficulties are then presented, focussing on reflections about the case of Fred. This case study is used to confirm some of Vygotski's ideas about teaching and learning phenomena that take place in interaction situations and promote the evolution of mathematical thinking in students with learning difficulties. The role of symbolic mediation and mediation by the entourage (pairs and the teacher) are also highlighted.