Experimental medicine, founded by Claude Bernard in the nineteenth century, decisively oriented medical research and especially modern biology. Amongst other things, it shed light on the role of the immune system, in other word, the means of defence developed by the body to fight microbes.Alain Fischer’s work, at the interface between genetics, immunology, and paediatrics, consists in identifying rare diseases’ genetic and molecular bases, inherited immunodeficiencies (IID), causing vulnerability to infection, auto-inflammatory and auto-immune diseases, and sometimes cancers.