What are Primary Immunodeficiencies (PIDs)?
Primary Immunodeficiencies (PIDs) also known as Inborn Errors of Immunity (IEI), are a large group of over 550 different genetic conditions where part of the body’s immune system is missing, doesn’t function correctly, or doesn’t function at all.
These conditions leave people more vulnerable to infections, autoimmunity, inflammation, and in some cases, increased risk of certain cancers. Most PIDs are caused by inherited genetic defects, though some arise spontaneously.
While PIDs are classified as rare diseases, they collectively affect a significant number of people worldwide, and many remain undiagnosed for years.
How Does the Immune System Work?
The immune system is the body’s defence network against infection. It is made up of cells, proteins and organs that work together to identify and destroy bacteria, viruses and other harmful invaders. Key components include white blood cells, antibodies, the lymphatic system, the spleen, and the thymus.
In PIDs, one or more parts of this system are missing or not working properly, leaving the body less able to fight off illness. Depending on which part of the immune system is affected, symptoms and severity can vary greatly from person to person.
Key Facts
- Over 550 distinct forms of PID have been identified.
- Approximately 6 million people worldwide are estimated to be living with a PID.
- Between 70–90% of those affected remain undiagnosed.
- PIDs are not contagious – they are mostly genetic conditions.
- Frequent, severe, or unusual infections can be a warning sign of an underlying immune disorder.
- Timely diagnosis reduces hospitalisations, prevents long-term complications, and improves quality of life.
- With appropriate diagnosis and treatment, many people with PIDs lead full, productive lives.
Who Does PID Affect?
PIDs can affect people of any age. Some forms are identified at birth or early in childhood; others may not become apparent until adolescence or adulthood, often after years of unexplained illness.
PID affects both males and females, across all ethnic backgrounds. Because symptoms can overlap with many other conditions, it is common for patients to spend years seeking a correct diagnosis.
See how PID affects people globally: The IPOPI PID Life Index is an interactive world map showing PID prevalence, diagnostic rates and access to treatment by country, including Ireland. Explore the map here
The 10 Main Categories of PID
PIDs are classified by the International Union of Immunological Societies (IUIS) into 10 main groups:
- Predominantly antibody deficiencies — the body doesn’t produce enough antibodies to fight infection
- Combined immunodeficiencies — multiple parts of the immune system aren’t working
- Combined immunodeficiencies with associated or syndromic features — as above but with additional physical features
- Diseases of immune dysregulation — the immune system attacks the body’s own tissues
- Congenital defects of phagocyte number or function — white blood cells can’t properly destroy bacteria and fungi
- Defects of innate immunity — the body’s first line of defence is impaired
- Auto-inflammatory disorders — repeated episodes of inflammation without infection
- Complement deficiencies — proteins that help fight bacteria are missing or reduced
- Phenocopies of PIDs — conditions that resemble PIDs but aren’t genetic in origin
- Inherited bone marrow failures — the bone marrow doesn’t produce enough immune cells
The full classification list is available from the IUIS website here.
IUIS 2025 IEI Classification Table – searchable list of all known conditions
The Impact of Living Without a Diagnosis
Without diagnosis and treatment, the consequences can be serious:
- Infants with severe forms of PID (such as SCID) may not survive early childhood without intensive medical intervention.
- Children may experience frequent illness affecting their education, development, and social life.
- Adults can suffer from chronic infections and complications that affect careers, family life, and mental health.
Useful Links
IPOPI — What are Primary Immunodeficiencies?
IPOPI PID Information Leaflets
