Introduction
The innate immune system recognises microbes by a set of receptors called pattern recognition receptors (PRRs). The "patterns" that these receptors recognise are structures found in a wide range of microbes collectively called pathogen associated molecular patterns (PAMPs). PAMPs are very often essential for the survival of the microbe. Some are, for example, involved in the uptake of nutrients, others are essential parts of the cell wall. PAMPs have therefore had to remain the same over time, despite constant evolutionary pressure to change in attempt to escape recognition and destruction by the immune system. By remaining unchanged over time, PAMPs are good targets for the non-changeable, non-adaptive innate immune system. In the following sections we are going to look a bit closer at what these PAMP structures are in different microbes and how the host cell recognises them.
We will also introduce the concept of "danger signals" as many times detection of a PAMP is not enough to activate the immune system. The immune system also needs "proof" that the host has been damaged, i.e. that the recognised microbe is in fact an invading pathogen. This proof is provided by the host cells as “danger signals”; signals from damaged self.