A silent army patrols every tissue. The immune system is the network of cells, tissues, and molecules that defends the body against infection, detects and eliminates abnormal cells, and participates in tissue repair.

Innate immunity provides rapid, non-specific responses; adaptive immunity learns specific threats and remembers them. White blood cells, lymphoid organs, antibodies, and cytokines interact in tightly regulated cascades. Overactivity produces allergy and autoimmunity; underactivity produces infection and certain cancers.

Vaccination educates the adaptive system without causing full disease. Immunosuppressive drugs are used deliberately in transplantation and autoimmunity but increase infection risk. Understanding immune function underpins much of modern infectious disease, oncology, and rheumatology practice.

• Body’s defense network against infection and abnormal cells
• Comprises innate and adaptive components
• Dysfunction leads to infection, allergy, or autoimmunity
• Targeted by vaccines and immunomodulatory therapies