The immune system is made up of specialised cells, proteins, tissues and organs that defend people against germs (bacteria, fungi, viruses, parasites, toxins, pollution) and some types of cancer.  It works to prevent infection and keep people healthy.  

Immunodeficiency (or immune deficiency) is where the immune system’s ability to fight infections is weakened or entirely absent.  Any deficit in the immune system can lead to frequent, prolonged or unusual infections and illness.  

A person who has an immunodeficiency of any kind can sometimes be referred to as being immunocompromised or having a weakened immune system.  

There are two types of immunodeficiency: 

Primary immunodeficiency (PID) are mainly genetic disorders. Most people with PID are born with the condition. Over 450 different PID conditions have now been identified and over 5,000 people in the UK are known to have a PID.  

Secondary immunodeficiency (SID) occurs when the immune system is weakened by another treatment or illness. SID is much more common as a health condition than PID. There are many potential causes of SID but the most common examples are blood or bone marrow disorders, drugs (medicines) and treatment for cancer. Some cancers can be responsible for SID, too. Although HIV infection is a cause of SID, this is usually treated as a separate problem, and it is often something that doctors wish to exclude before considering other causes for primary or secondary immunodeficiency.  

This section gives you information about PID and SID, including diagnosis, treatments, advice on keeping well and patient stories.