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 one of the two major divisions of the immune system, along with the innate immune system. As we learned in the first article of this miniseries, innate immunity is responsible for the first line of protection against pathogens.