An immune system is present in all plants and animals. We know this because biologists have found genes coding for toll-like receptors in many different metazoans. These toll-like receptors can recognise bacteria as 'foreign', and are the starting-point for immune reactions. The type of immunity which is triggered by the toll-like receptors is called innate immunity. This is because it is entirely inherited in our genome, and is fully working as soon as our tissues and organs are properly developed.