What is the immune system?
The immune system is the body's defense network against germs such as bacteria, viruses and fungi. It includes white blood cells, antibodies, the lymph system and organs like the spleen and bone marrow.
The immune system has two main parts. The innate immune system responds quickly to any invader. It includes barriers like skin and mucus, inflammation and white blood cells that swallow germs. The adaptive immune system takes longer to respond but targets specific germs, using B cells to make antibodies and T cells to destroy infected cells.
After fighting an infection, the adaptive immune system keeps memory cells. If the same germ returns, the body responds faster and more strongly, often before you feel sick. Vaccines work by training the immune system with a harmless version or part of a germ.
When it goes wrong
Sometimes the immune system overreacts to harmless things, causing allergies, or attacks the body's own tissues, causing autoimmune diseases such as type 1 diabetes and rheumatoid arthritis. A weakened immune system, from illness or certain medicines, makes infections more likely.
Did You Know?
The thymus, a small organ in the chest where T cells mature, is largest in childhood and slowly shrinks with age.
This answer is general information, not medical advice. For a personal health question, talk to a doctor.