Immune system

Human Immune System

The human immune system is made up of different components that allow labelling and elimination of foreign substances, as well as the regulation of these processes.

The functioning or malfunctioning of this system is therefore of vital importance to health and disease. Not only does it constitute the body's defensive system against foreign antigens, but a malfunctioning of its regulatory processes can result in conditions such as autoimmune disease. A major area of interest is also the role of its different components in treatments to conditions such as leukaemia, where Graft vs Host Disease due to an inflammatory response launched against the donor's haematopoeitic transplant is a hurdle still to overcome.

Components of the Immune System

Physical Barriers

The first line of defence are physical barriers that prevent the pathogen or foreign substance from entering the body or, once entered, from harming it.

The skin is one of the most obvious exmaples. Not only does it provide physical protection from invasion, but the presence of antimicrobial substances and a low pH prevents pathogenic bacterial colonisation for example. As well as on the skin, the large intestine for example contains colonies of normal flora, non-pathogenic bacteria that provide a competitive environment for pathogenic strains.

Epithelial lining of the respiratory and digestive tracts is the physical barrier found in these systems protecting the host from infection spreading into the blood, from where it would circulate throughout the body and lead to septicaemia (blood poisoning).

Mucociliary clearance is another example of the body's ability to combat invading foreign substances, which are trapped in the mucus transported along by the cilia found on respiratory epithelial cells.

Lysozyme in tears helps defend the eye, which constitutes a gateway to the paranasal sinuses, from bacterial invasion.

Innate Immune Response

The human immune response can be categorised into the Innate and Specific Immune Responses.

On initial contact with a pathogen or other foreign substance, the body's Innate Immune Response is launched, which can again be subdivided into Humoral and Cell Mediated Immunity.

The first discribes all the components of the Immune System that are found in the serum and their mechanisms. These include cytokines, low molecular weight proteins that are released by cells of the immune system to interact with other cells to produce an inflammatory response and therefore contribute greatly to regulation of the same. An example of a cytokine involved in the innate immune response is IL-6 (Interleukin-6), which is secreted by macrophages after phagocytosis of a bacteria for example.

Complement is another major component of the body's Humoral Innate Immunity. When a foreign antigen binds to a freely circulating antibody in the blood, such as IgM, a cascade of reactions is initiated involving serum and membrane proteins leading to the attraction of immune cells to the site of infection, as well as opsonization of the pathogen with complement proteins, which prepares the cell for phagocytosis.

Cell Mediated Innate Immunity is comprised of the body's cellular response to infection. As already mentioned, phagocytes such as macrophages and neutrophils engulf foreign substances and destroy them intracellularly, displaying the foreign antigens on the cell-surface for recognition by T-cells for example and the initiation of events leading to Acquired Immunity. The release of cytokines by phagocytes leads to an amplification of the immune response and attraction of other, more specialised immune cells, such as B-lymphocytes. Binding to the displayed antigen results in activation of the B-cell and the production of antibodies specific to that antigen.