The invention concerns a composite support comprising a base support for the treatment of a biological fluid, in which: " the base support is essentially- constituted by a first polymer carrying anionic or anionizable groups； " at least a part of the surface of the base support is coated with a second polymer ionically bonded to the first polymer, the second polymer carrying the cationic or cationizable groups which are capable of forming an ionic bond with the anionic or anionizable groups of the first polymer； in which the second polymer is in the colloidal form and in mixture with a polyacid during application to the support, allowing the composite membrane to adsorb at least one entity containing anionic or anionizable groups by bonding with cationic or cationizable groups of the second polymer.
