The immune system is the collection of cells, tissues, and molecules that work together to recognize the healthy cells that make up the body, and protect us against the unfamiliar or damaged. 

The immune system monitors our body continuously searching for certain categories of things that may threaten our health: infectious microbes, viruses, fungi, and parasites (i.e., germs or pathogens); toxic cellular products; and damaged or diseased cells, including senescent or tumor cells.

The adaptive immune system is responsible for the more complex and optimized immune responses that develop when innate immunity is insufficient to manage a threat. It is the specific immunity we acquire over time as the immune system is challenged with new antigens and learns to deal with them.