Immune System
The network of cells, tissues and proteins defending the body against infection.
What it means
The immune system defends against infection and abnormal cells. It has two arms:
- Innate immunity — immediate, non-specific defences including skin, mucous membranes and inflammatory cells
- Adaptive immunity — slower but specific, producing antibodies and memory cells that respond faster on re-exposure. This is the basis of vaccination
It can fail in either direction. Underactivity permits infection; overactivity or misdirection produces allergy, where harmless substances are attacked, and autoimmunity, where the body's own tissue is.
In practice
"Immunosuppressed" and "immunocompromised" describe reduced immune function, whether from disease, chemotherapy, long-term steroids or immunosuppressant medication.
The distinction matters clinically: immunosuppressed people need earlier assessment of infection, and some vaccines are contraindicated.
Why it matters
Anyone immunosuppressed who develops a fever should be assessed urgently rather than observed, as infection can progress rapidly and the usual signs may be blunted.
Autoimmune conditions frequently coexist, so a diagnosis of one raises the likelihood of another.
Clinically reviewed by Dr Mohammad Zubair Khan, GMC 7563469
Last reviewed
August 30, 2026