Infodump 2
Infodump 2
As we previously established in the introduction, the immune response against an invading pathogen is an intricately orchestrated cascade of events involving numerous cell types and molecules. This ‘Immune response’ is actually a result of the activity of two organized systems: Innate immunity and Adaptive immunity.
Innate immunity refers to the non-specific mechanisms that an invading pathogen faces when trying to infect the host system. All multicellular organisms (plants, invertebrates and vertebrates) have evolved these mechanisms for protection against common invaders. Hence the name, ‘Innate’ immunity. These mechanisms are encoded in the germline and are evolutionarily more primitive. It consists of physical barriers, chemical components and certain cell types.
For a successful infection, the pathogen must first breach the physical barriers. The physical barriers skin and the mucous membranes. If the entry of the pathogen is through the digestive tract, it must also survive the enzymes and the acidic pH of the stomach. If the pathogen manages to enter the host, it is recognized by the cells of innate immunity, mainly phagocytes. Phagocytes recognize foreign invaders by identifying PAMPs (Pathogen-associated molecular patterns) which are common chemical structures found on and unique to classes of pathogens and not found in mammals. For example, encapsulated bacteria express a certain type of polysaccharide that is not found on other types of bacteria or humans. These PAMPs are identified by PRRs (Pattern Recognition Receptor) expressed on white blood cells and a cascade of events to eliminate the foreign organism is triggered. This recognition is non-specific but allows for quick detection of invading pathogens. PRRs are germline-encoded and are highly conserved. Innate immunity also includes a set of serum proteins known as the complement system that binds and ultimately leads to the destruction of the pathogen. This system will be described in detail in future infodumps.
Most of the pathogens are eliminated by the innate immunity within a matter of hours of invasion.
Innate immunity is swift in eliminating most pathogens. However, several infecting microbes have evolved to evade these mechanisms. To eliminate the pathogens that have circumvented the innate immunity, adaptive immunity is activated. As the name suggests, this system ‘adapts’ in real time to the infection to successfully eliminate the invading organism. Adaptive immunity is a feature exclusive to vertebrates. It consists of two mechanisms: cell-mediated immunity and humoral immunity. Humoral immunity utilizes antibodies secreted by B-cells to combat pathogens. These antibodies attach to antigens of the pathogen which labels it for destruction by phagocytes or hinders host-pathogen interaction thus neutralizing it. These antibodies stop the pathogen from entering cells. However, it has no access to pathogens already inside the host cell. Cell-mediated immunity is the activation of T-cells which have the ability to eliminate intracellular pathogens by identifying and destroying infected cells.
Pathogen recognition by B and T cells is highly specific, unlike the innate immunity. Even a small variation in the antigens can trigger different responses. Adaptive immunity contains a large population of cells containing multiple copies of a unique receptor on its surface. This is achieved by a strategy known as ‘Generation of Diversity’. The genomic DNA is edited and arranged to yield an array of receptors which theoretically can identify any antigen that is encountered which is nonself. These cells are then screened in the primary lymphoid organs to eliminate any faulty cells and then released into circulation.
Innate and adaptive immunities are not isolated systems. Innate immunity ‘helps’ the adaptive immunity to carry out its proper functioning. The antigens are presented to the B and T cells for recognition by cells of innate immunity thereby triggering their function. Certain molecules called the ‘Cytokines’ are released by the innate cells upon interaction with the pathogens that signal the B and T cells to perform new functions, such as synthesis of other soluble factors or differentiation to a new cell type. Cytokines and their functions will be described in a future infodump.
The next infodump will focus on the various cell types of the immune system.