The human body regularly encounters harmful micro-organisms, and because of this it has developed a system of defences to help identify and eliminate infective pathogens in the body, known as the immune system.
Humans have two types of immunity: innate immunity and acquired immunity.
- innate immune system (also known as the non-specific immune system) is the frontline of defence against potentially harmful pathogens. Mediators of the innate immune system include phagocytes such as dendritic cells and macrophages.
- acquired immune system (also known as the specific immune system) deals mainly with infections in the later stages, as well as forming what is known as “immunological memory”, which helps the body to remember certain pathogens and how to combat them. Acquired immunity is unique due to its specificity. It develops from lymphocytes that have receptors which are antigen-specific.
Innate immunity is not altogether non-specific, as it has the ability to discriminate between normal cells and a wide range of pathogens. The innate immune system has the ability to recognise these pathogens through pattern recognition receptors (PRRs). These PRRs are small in number, when compared to the many receptors used by the acquired immune system
All PRRs share the same characteristics. Firstly, a major characteristic of PRRs is the ability to pick up components of microbes knows as pathogen-associated molecular patterns (PAMPs). PAMPs are vital for the survival of these micro-organisms. Because of this, they are difficult for the micro-organism to change. Secondly, these PRRs can detect pathogens, regardless of which stage of the life cycle they are in. Thirdly, PRRs are "germline encoded, non-clonal, expressed on all cells of a given type, and independent of immunologic memory” 1. Each PRR will react with its specific PAMP, will have its own expression pattern, and activate their specific signalling pathway and antipathogen response.