The main purpose of the immune system is to protect the body from harmful pathogens, such as bacteria, viruses, fungi, and parasites. It functions as a complex network of cells, tissues, and organs that work together to detect and respond to these invaders. When a pathogen enters the body, the immune system acts by identifying and neutralizing the threat, preventing illness and disease. It has both innate defenses, which are immediate and non-specific, and adaptive responses that are more precise and improve with exposure to specific pathogens, leading to immunity.