Innate immunity is that which reflects our immediate biological defense against the outside world. It is characterized by discrimination of what is self versus nonself, or harmless versus dangerous. Innate immunity does not require prior sensitization ('memory') nor does it generate an augmented secondary ('recall') response. Many innate immune receptors are pattern recognition receptors (PRR's) that have evolved over millenia to recognize common pathogens in their simplest form, subunits known as pathogen-associated molecular patterns (PAMP's); these obligate pathogen-derived substances cannot easily be altered by the pathogen without loss of viability. In other instances, innate immune receptors recognize self ligands that may be displayed in a modulated fashion upon infection or transformation to notify the immune system that something is wrong. Cells of the innate immune system include macrophages and dendritic cells [the "professional" antigen-presenting cells (APC's)], polymorphonuclear leukocytes, and certain lymphocytes, such as natural killer cells, and even a repertoire of B cells that is fixed at birth. Cross-talk between the innate immune system and the adaptive immune system is critical for the mounting of an effective sterilizing immune response, such that without innate immunity, we cannot exist in nature.