Final answer:
The immune system is responsible for defending the body against harmful agents such as bacteria and viruses. It is divided into the innate immune system, which provides nonspecific defense, and the adaptive immune system, which provides specific, targeted responses with memory for future protection.
Step-by-step explanation:
The immune system is the body's defense against bacteria, viruses, and other harmful agents. It comprises a complex network of cells, tissues, and organs that work together to protect the body from potential threats. This system can be categorized into two major types: the innate immune system, which offers a nonspecific defense and acts as the first and second line of defense, and the adaptive immune system, which provides specific, targeted responses and includes a memory component to enhance protection upon reexposure to pathogens.
The first line of defense consists of physical and chemical barriers like skin and mucus, which are always ready to protect the body from infection. The second line of defense involves processes such as inflammation and phagocytosis. Finally, when a pathogen penetrates these initial layers of defense, the third line of defense, or the immune response, is activated. This specific response is part of the adaptive immune system and can remember the pathogen, thus providing faster and stronger protection if the same pathogen invades again.