[How should an immunodeficiency be explored?].

When immunodeficiency is suspected, simple investigations allow to direct the diagnosis. Blood cell count brings essential elements: the lymphopenia, signing a deficit of cellular immunity, is often ignored because of the physiological hyperlymphocytosis of the young child. Only a neutropenia lower than 500/mm3 could be responsible for obvious clinical signs. The level of the serum immunoglobulins IgG, IgA, and IgM becomes interpretable only after the age of 4-5 months. In most of the cases, all the elements brought by these simple laboratory investigations jointly to those brought by the medical history and the clinical examination make it possible to target the more specific investigations required in a second time, according to the type of evoked immunodeficiency.