The human body has three lines of defense against pathogens. The first line includes physical and chemical barriers like the skin and mucous membranes. The second line involves defensive cells and proteins that attack pathogens that breach the first line. The third and most complex line is the immune system, which recognizes and mounts reactions against pathogens. It consists of both innate and adaptive immunity using mechanisms like phagocytosis, antibodies, and lymphocytes to fight infection.