Overview of innate and adaptive immune systems.

The innate immune system includes defense mechanisms encoded in the germline of the host. It does not require prior sensitization to antigen, and represents a primitive line of defense, closely linked to mucosal or barrier immunity. The adaptive immune system is based on prior sensitization to antigen and antigen presentation by dendritic cells in the context of MHC. Because it acts based on antigen-specific receptors on T cells and B cells, it has greater specificity.