The immune system is the body’s complex defence network — the cells, tissues, and molecular products that distinguish self from invader and destroy the latter. Its organs (thymus, spleen, lymph nodes, bone marrow), its cells (lymphocytes, phagocytes, dendritic cells), and its weapons (antibodies, complement, cytokines) mount layered defences: innate barriers and inflammation first, then the precise, remembering adaptive response. Nutrition and immunity are inseparable — protein-energy malnutrition cripples defences, while vitamins A, C, D, zinc, and iron each play defined immune roles. Vaccines train the system; immunosuppressive drugs restrain it. It is the most sophisticated defence system known — and the reason a sterile world would still not keep us safe.