Life is a never-ending battle against infectious micro-organisms. Most of the time, this struggle rages below the radar screen of our awareness as our body’s many innate defense mechanisms prevail keeping us in a state of relative health. However,occasionally these defenses are breached, typically by pathogens that have evolved specialized strategies to subvert or avoid rejection by hard-wired innate immune mechanisms. Such infections require more sophisticated and flexible defense strategies afforded by lymphocytes that make up our adaptive immune system.Adaptive immune responses are characterized by the capacity to recognize and remember pathogen-specific antigens.Although these general principles are now well-established, many of the all important fine details are still incompletely understood. What are the rules that govern the priming of CD4 and CD8 T cells by antigen-presenting cells in vivo? What are the signals for Bcell activation and differentiation? Once an antigenic stimulus has been received, what determines a lymphocyte’s further career choice(s)?.. all these questions were tried to answer in present book.