In earlier units we discussed how the immune system provides different ‘layers’ of protection focusing mainly on immune barriers. Now we will turn our attention to how innate and adaptive immunity function. We will simplify the processes of innate and adaptive immunity into two stages, recognition and response. So far we have described the immune system as a series of barriers that protect the host from microbes. In this unit, the students will explore the immune response that takes place when a microbe penetrates the sterile environment of the host.