All universities will have an extenuating circumstances policy that your son or daughter should look up. This policy is there to assist students who may encounter significant personal difficulties outside their control such as ill health. Many universities can put into place adjustments to support study and assessment.
It is also worth your son or daughter making an appointment with their course leader to explain their circumstances as soon as possible into the year or even before the course starts. If their course teachers know their circumstances from the start they can put in place structures to help them. This can all be done in confidence.
It may be worth you taking information about the condition to this meeting so they have all the information they need. Immunodeficiency UK has resources on conditions and education that can be downloaded here.
It is also worth noting that people with an immunodeficiency are covered under the Equality Act 2010. This means protection under the law from unfair treatment relating to a medical condition. Read more here.