Researchers at the Royal Free hospital have made a significant discovery in ensuring people are able to fight infections.

Professors at the UCL Institute of Immunity and Transplantation (IIT), based at the hospital in Pond Street, discovered a faulty copy of a single gene – CTLA4 – leads to the condition primary immunodeficiency (PID).

The research will mean doctors can diagnose this condition more easily, using a simple genetic test.

Patients with PID have an immune system which does not provide them with enough protection from infections.

As a result they can suffer from a range of symptoms, including heart problems, repeated bouts of severe infections like pneumonia and skin abscesses.

They can also suffer from autoimmune symptoms, where the poorly regulated immune system begins to attack the body.

The researchers – Professor Bodo Grimbacher, Professor Lucy Walker and Professor David Sansom – say the discovery was only made possible because the institute is based at the Royal Free where “researchers, clinicians and patients are joined together on one site”.

It is an argument the medical staff are hoping residents in Hampstead and councillors at Camden Council will take on board as they look to get permission to build a new £42million building next to former church St Stephen’s in Rosslyn Hill.

The discovery was published in the journal Nature Medicine.

Professor Walker said: “This is a really important discovery. We now understand why there is a problem with the immune system in some patients with PID and that will enable us to develop better treatments.”

Professor Sansom added: “It was important that we had already published papers on the function of this gene, so we had a lot of information about the protein this gene produces and its role in the immune system.

“Now we have a clearer understanding of what it does in the immune system in people.”

Although doctors have long known that PIDs are genetic disorders, no one has previously identified a role for CTLA4 in this condition.

This new work has revealed people need two healthy copies of CTLA4 for their immune system to function properly.

It also means it could be easier to treat patients with PID. Some patients with PID could be given abatacept, a drug used to treat patients with arthritis, because this drug plays a similar role to the body’s natural CTLA4 and suppresses autoimmune symptoms.

Dr Siobhan Burns, an immunodeficiency clinician at the Royal Free Hospital, said: “This discovery has been possible only because of the way the institute places clinicians and researchers close together. The scientists have provided their expertise in terms of identifying the correct gene and looking at its function, while the clinical team have access to the patients.”