Abstract
In this chapter, we will provide an overview on the roles of B and T cells in the response of the adaptive immune system to microbe invasion. In this context, we will discuss the impact of memory cells over human lifespan. Adaptive immunity is able to respond more specifically than the innate immune system, because it uses with TCRs and BCRs (that develop into antibodies) very specific antigen receptors. For an understanding how the diversity of these antigen receptors is created based on very few genes, we will discuss lymphocyte maturation, clonal selection and the mechanisms of somatic recombination.