The immune system is divided into innate and adaptive immunity. Either immunity consists of humoral and cellular responses, and immunity is maximized when both responses coordinately function. Adaptive immunity has been intensively studied, while it was only recently that we gained some understanding of innate immunity. In particular, cellular responses in innate immunity have been poorly understood compared with humoral responses. In addition, the mechanisms and roles of innate immune responses could be distinct between the organisms that possess both innate and adaptive immunity and those possessing only innate immunity. On the other hand, invading pathogenic microbes employ various strategies to inhibit the host immune system for their survival. I here summarize what needs to be known to gain a deeper understanding of the innate immune response. The readers are suggested to refer to the accompanying articles for more detailed description.