The immune system consists of two parts that work in different ways and interact with each other to protect against infection. Each part is made up of different cell types and pathways. The first, called innate immunity (or natural immunity), provides protection against all kinds of infection in a nonspecific manner. This form of immunity is present in plants, fungi, and animals and is an evolutionarily early form of immunity. It is considered innate because it does not depend on prior exposure to a pathogen.

By contrast, the second form of immunity, called adaptive immunity (or acquired immunity), is specific to a given pathogen. This form of immunity “remembers” past infections, meaning that subsequent encounters with the same pathogen generate a stronger response. In other words, the immune system adapts over time, and immunity is acquired after an initial exposure. Adaptive immunity evolved later than innate immunity and is unique to vertebrates. Table 43.1 summarizes the major differences between these two forms of immunity.