This document provides an overview of the immune system and how it responds to microbial infections. It discusses both the innate and adaptive immune responses. The innate response involves inflammation, complement activation, NK cells, and activation of antigen presenting cells. The adaptive response involves activation of naive T lymphocytes, polarization of T cells into Th1 and Th2 subsets, roles of Th1 and Th2 cells, and roles of memory B and T cells in providing long-term protective immunity. The document also briefly discusses some methods microbes use to evade the immune system.