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The immune system is the body's complex defence network, responsible for protecting us against disease. Its primary job is to identify and eliminate pathogens - disease-causing microorganisms such as bacteria, viruses, fungi, and parasites - as well as other harmful substances or abnormal cells, like cancer cells. It's a dynamic system involving various organs (like lymph nodes, spleen, thymus, bone marrow), specialised cells (mainly white blood cells), and proteins circulating throughout the body. The immune system can distinguish between the body's own 'self' components and foreign 'non-self' invaders.