In a healthy immune system all 3 levels of the immune system are functioning efficiently.
Innate Immune System
The innate immune system comprises of the first and second lines of defence, this is the most primitive of the immune systems and works in a non-specific way.
First line of defence
The first line of body's defence against pathogens starts with its passive barriers. This level restricts foreign particles from entering the body.
These barriers are passive because they do not detect pathogens, or react to their presence.
Examples include:
The first line of body's defence against pathogens starts with its passive barriers. This level restricts foreign particles from entering the body.
These barriers are passive because they do not detect pathogens, or react to their presence.
Examples include:
Second line of defence
If the pathogens are able to get past the first line of defence, and enter the body the second line works to remove these pathogens in a non specific way. The second line of defence is a group of cells, tissues and organs that work together to protect the body.
Antigen-Presenting cells such as macrophages engulf and destroy foreign substances that enter the body's tissue. Macrophages are cells derived from monocytes. Macrophages and other antigen presenting cells exit in the bloodstream and enter body tissues to patrol for pathogens. When the macrophage encounters a foreign substance, it engulfs it, partially digests it, and the antigenic fragment binds to a class II MHC marker.
Shown left:
Antigen-Presenting cells such as macrophages engulf and destroy foreign substances that enter the body's tissue. Macrophages are cells derived from monocytes. Macrophages and other antigen presenting cells exit in the bloodstream and enter body tissues to patrol for pathogens. When the macrophage encounters a foreign substance, it engulfs it, partially digests it, and the antigenic fragment binds to a class II MHC marker.
Shown left:
More specifically the second line of defence is involved in the inflammatory response involving a different group of cells particularly mast cells which release histamine. Histamine is a signalling molecule which causes the increased permeability of blood vessels leading to an increased concentration of white blood cells like macrophages and neutrophils in the area needed to remove foreign substances.
An example of this is the allergic response.
Interferon: (non -specific)
Interferons are proteins made and released by host cells in response to the presence of pathogens such as bacteria, viruses, tumor cells or parasites. Interferons only target virues and allow for communication between cells to trigger the protective defenses of the immune system that eliminate pathogens or tumors.
•Antiviral chemical is released from a host cell when it is colonised by a virus.
•Interferon spreads to neighboring cells, which take up and begin producing anti viral enzymes.
•These corrupt the viral DNA preventing the host nucleus from making more copies of the viral DNA.
•This inhibits the virus from colonising nearby cells and spreading further.
Complement: (non -specific)
Complement are proteins that act on bacteria. They assist phagocytes to recognise foreign antigens by attaching to the pathogen and acting as a cell wall. When the bacteria cell contents spill into the host more macrophages are stimulated.
Adaptive Immune System
The adaptive immune system comprises of the third line of defence which acts against antigens in a specific way. It is the level of the immune system which is involved in immunity.
Third level of defence
The third line of defence is a specific response to a recognised antigen. This is carried out by either the cell mediated - T cell response or humoral - B cell response.
Cell mediated response:
An antigen is detected by a T(Helper) cell, the activation that occurs when this complex forms causes T helper cells to release interleukin 2 to activate cytotoxic T cells and immature B cells.
The body contains millions of different T-cells, each able to respond to specific antigens.
Cytotoxic T cells are able to recognise and induce apoptosis in the infected cell, cytotoxic T cells also release perforins which cause perforation in infected cells also leading to their death.
Humoral response:
- T helper cells stimulate immature B cells to divide, forming B plasma cells- that are able to produce antibodies, and memory B cells.
- Memory B cells are soley responsibile for immunity, once an antigen has been encountered the B cell will remain in circulation for years to rest of life. Should the same antigen be encountered again, the B memory cells would quickly be stimulated for a second response.
- B plasma cells are the antibody factories of the immune system, when an immature B cell has been stimulated it causes proliferation of many B plasma cells. The high concentration of antibodies in the immune system allows the pathogen to be overcome.
Humoral immune response |
The immune response to a pathogen |