Final answer:
The adaptive immune system is slower than the innate system on first exposure to a pathogen because it needs to specifically identify the pathogen and generate a targeted response, a process that involves time-consuming steps of immune cell proliferation and specialization.
Step-by-step explanation:
The immune system is critical in protecting the body against pathogens, and it comprises two main types of defenses: the innate and the adaptive immune systems. The innate immune system responds quickly but in a non-specific manner to all pathogens, employing barriers like skin and mucous membranes, and actions including phagocytosis, inflammation, and activation of the complement system. On the other hand, the adaptive immune system takes longer to react upon the first encounter with a pathogen because it involves a complex process of identifying the pathogen's specific antigens and then generating a targeted response. This process includes the creation and proliferation of lymphocytes that can recognize the pathogen, which takes time. However, because the adaptive immune response is specific, it not only effectively clears the pathogen but also creates memory cells that ensure faster and more effective responses upon subsequent encounters with the same pathogen.