This may be a good way to go, but it really would be best to first find out whether your 4 year old son has an immune deficiency or not. He may well have one or other of the various kinds of immune deficiency, particularly an immunoglobulin deficiency. It is recommended that your son is seen by a paediatric immunologist. If you are in Gauteng or KZN, I can highly recommend Dr Andre van Niekerk who is at the Clinton Clinic in Alberton and also works at the Alberlito Hospital in Ballito.
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