Although we discussed the different components of the immune system one at a time, they in fact work together to provide a coordinated response to pathogens. We have seen how B cells often require T cell help, how the complement system can be activated by antibodies produced by B cells, and how macrophages present antigens in association with MHC proteins to T cells. In turn, pathogens have evolved means of evading the immune system. In this section, we consider the flu virus, the bacterium that causes tuberculosis, and the malaria parasite. These three pathogens illustrate how pathogens evolve and adapt in response to the immune system, and how the immune system in turn evolves and adapts in response to pathogens.

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