Hi welcome to HCM I have gone thru regarding high WBC count in blood in a child . I can understand your concern . Dear, your doctor is right to send your son for more tests to check for glandular fever , as he already has history of immune problem & had BMT. It is usual thing to be done to just repeat the blood work in a few weeks to make sure that the count is under control . For your info ,There are a few different causes of an elevated white blood cell count. Since the white blood cell are involved in fighting off infections in the body. Doctor orders routine checkup blood work that comes back with a high white blood cell count ,this typically just means that your son has just gotten over some viral infection (like a common cold) In rare cases, a high white blood cell count can be a sign of a serious problem, most typically a blood cancer like a leukemia or a lymphoma. Even though this is not at all common, this is the reason that your pediatrician is being cautious and checking additional blood work. It will be important to followup closely with them to make sure that everything turns out normal and , if not , the right treatment can be given at right time . So you need not worry , cooperate with your doctor .And If feel any doubt instantly consult another doctor for second opinion . A balanced diet containing essential nutrients - fiber, vitamins,minerals, antioxidants as , curd ,Omega 3, flax seed , oranges , coconut water , Aloe era juice +Amla juice and regular physical activity and Deep Breathing- Inhale- Hold -Exhale - Hold 10 -20 times ,all help strengthening immune system and assure healthy life . Hope this helps solve your query .Take care , All the best . Don t hesitate to get back if have any further query