Final answer:
The immune system is a network of cells, tissues, organs, and chemicals that fight off pathogens. It includes the innate and adaptive immune systems, which provide non-specific and specific responses to pathogens, respectively.
Step-by-step explanation:
The immune system is a complex network of cells, tissues, organs, and chemicals that fight off pathogens. It can be divided into two types of defense systems: the innate immune system, which provides a quick but non-specific response, and the adaptive immune system, which provides a specific response and includes a memory system.
The innate immune system includes physical barriers such as the skin and mucus, chemical and biological barriers, inflammation, activation of the complement system, and non-specific cellular responses like phagocytosis.
The adaptive immune system, on the other hand, mounts a highly specific response to substances and organisms that do not belong in the body and has a memory system that allows it to respond with greater intensity upon reencountering a pathogen.