CD40LG and inborn error of immunity: In this primary immunodeficiency, there is a congenital arrest of B-cell differentiation at the pre-B cell stage in the bone marrow, and affected patients, usually with an inborn severe decrease in mature B cells and consequently plasma cells, manifest with markedly decreased numbers of peripheral B cells (<1%) and consequently show very low or absent IgG, IgA and IgM [23].