Clinicians from the Royal Free London's immunology service spoke to BBC Radio 4 last week when immune deficiency featured as the main topic on its Inside Health programme.
Dr Siobhan Burns, clinical lead for immunology, described how this rare and important diagnosis is often missed and how a lack of treatment causes endless infections and damage including irreversible lung scarring.
During the programme Dr Burns gave an easy to understand explanation of how the immune system works and what treatment entails. Patients usually have a normal white blood count, but doctors rarely ask for a further test which checks exactly which infection-fighting cells are present, even though the test is cheap.
Andrew Symes, clinical nurse specialist in immunology, said that some of his patients reported having had years of infections before being diagnosed.
Listen to the full programme on BBC iPlayer Radio. The piece will be available until October 2015. The item starts 10 minutes into the programme and lasts for 12 minutes.