This MOOC is aimed to offer the most up-dated information on the cellular and molecular players of innate immunity, and on the mechanisms that lead to the elimination of the pathogens, of either bacterial, viral, fungal or parasitic origin. Accordingly, strategies of the pathogens against the innate immune system are described. The fact that innate immunity is under influence of the genetic evolution, and the microbiota is addressed as well as its crosstalk with the nervous system. Its links with adaptive immunity is also presented.