THE INNATE IMMUNE SYSTEM
The innate immune system consists of various non-specific processes that ensure its effectiveness against foreign bodies. Such processes include physical and chemical barriers, however cellular processes are largely responsible for non-specific elimination of any pathogens in the event of the previous barriers being breached, or the prevention of the barriers being threatened at all.
Types of leukocytes involved in innate immunity
Mast cells - allows recognition of harmful antigens by binding to pathogens directly, and express a variety of toll-like receptors that activate immune cell responses.
Phagocytes - specialised cell capable of engulfing and absorbing pathogens
Macrophages - A type of white blood cell that surrounds and kills microorganisms, removes dead cells, and stimulates the action of other immune system cells
Neutrophils - Trap and kill invading pathogens
Eosinophils - Specialized pro-inflammatory white blood cells
Natural Killer cells - Lymphocytes that respond quickly to a wide variety of pathological challenges. Additionally, NK cells secrete cytokines such as IFNγ and TNFα, which act on other immune cells like Macrophage and Dendritic cells to enhance the immune response.
Basophils - triggers the release of histamines when encountering an antigen. Furthermore, the basophil can also attract eosinophils and neutrophils when required
Monocytes: Initiates inflammation upon detection of potential pathogenic attack from the outside of the human body
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Note: Although not a leukocyte (white blood cell), it is important to also make reference to histamines, which increases the permeability of the capillaries to white blood cells and some proteins, enhancing the efficiency and effectiveness of the immune system.
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The Epidermis
The epidermis is a seemingly insignificant aspect of the innate immune system; however it plays a vital role in the prevention of infection on all humans as the initial barrier between bacteria and the human, generally acting as the first line of innate immune defence against infection. The stratum corneum, exhibited in the diagram below, is the outermost epidermal layer, comprised of layers of dead cells, keratin, and very resilient specialized skin cells.
Periodic shedding of epidermal cells helps remove microbes from the skin's surface, which is also slightly acidic and provides additional protection against many microorganisms that are unable to survive and reproduce on such environments. Furthermore, on the surface of the skin a protective film of sebum is formed, as produced by the sebaceous glands of the skin, resultingly inhibiting the growth of certain pathogens. In addition, the epidermis also produces sweat from respective sweat glands, which serve as a chemical barrier containing specific enzymes, such as lysozyme, which is also present in tears and saliva, that simultaneously kills any present pathogens, and allows for adequate thermoregulation. Keratinocytes constitute approximately 90% of human epidermal cells, and express toll-like receptors which detect conserved molecules on pathogens and trigger an inflammatory response.
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Furthermore, dendritic cells function as immune cells that link innate and adaptive immunity. The main purpose of these innate cells is to capture, process, and present antigens to adaptive immune cells. Similarly to dendritic cells, mast cells are both innate and adaptive immune cells that recognize harmful antigens by binding to pathogens directly, and express a variety of toll-like receptors that activate immune cell responses. Mast cells have a significant role in physiological, pathological, as well as in defense mechanisms in response to pathogens.
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The Mucosal Immune System
The mucosal immune system is especially vital for infectious agents that are ingested or inhaled, as this bypasses other first-line defences (such as the epidermis)
A significant portion of immune-related proteins are located in mucus gels and the mucin-rich surface layers that line many tracts and structures of the body; mouth, nose, eyelids, trachea, lungs, stomach, and intestines. Such proteins are responsible for locating, immobilising, and/or killing pathogens, whilst simultaneously suspending them within the mucus. This mucus is then moved via muco-ciliary clearance or by coughing/sneezing.
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The mucosal immune system is mostly comprised of three significant processes;
- Goblet cells, which serves to secrete mucin and create a protective mucus layer.
-Ciliated cells, with its surface surrounded by cilia, which are tiny hair like structures on the surface of the cell that sweep hair, mucus, trapped dust and bacteria up to the back of the throat where it can be ingested
-Basal cells, which are located at the bottom of the epidermis, and produce new skin cells which replace and move older cells toward the surface, where they die and create a protective layer that also defends the human body from pathogenic attack.
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The Inflammatory Response
The inflammatory response is the body’s natural reaction to injury and infection, and has three phases:
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During the inflammatory response, damaged cells release chemicals including histamine, bradykinin, and prostaglandins. The inflammatory response plays a vital role in immunity. When tissues are damaged, the inflammatory response is activated, thus mobilising the immune system. By vasodilating, blood flow to the site of injury is initiated, allowing the immune cells such as neutrophils (which trap and kill invading pathogens.) and eosinophils (specialized pro-inflammatory white blood cells) to efficiently and effectively travel to the site of damage. The lymphatic system is also vital for providing optimal function of our general and specific immune responses, by utilising lymph nodes to monitor the lymph flowing into them and produce cells and antibodies which protect our body from infection and disease.
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The inflammatory response is the body’s natural reaction to injury and infection, and has three phases:
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Phase 1 - Acute:
Inflammation during this stage is fast and painful. It includes the vasodilation of blood vessels, and the transfer of fluid into surrounding tissue. This causes additional damage to the injured area, as function is decreased due to large swelling. New blood cells begin to be developed, and the area is swollen, red, and painful.
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Phase 2 - Repair:
The body begins to fix the damaged site. The white blood cells clean up the debris from the injury and new body tissue begins to be formed.
This may potentially produce scar tissue during the reparation of the injury, which needs to be minimised for proper healing to occur, as scar tissue is weaker than normal tissue.
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Phase 3 - Remodelling:
The remodelling phase of the inflammatory response continues to rebuild the injured area.
This includes further production of scar tissue, alongside the development and strengthening of new functional body tissue (which can replace scar tissue with appropriate treatment). Red blood cells help create collagen; tough, white fibres that form the foundation for new tissue.
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The Complement System
The complement system is an extremely significant component of innate immunity, representing one of the major effector mechanisms of the innate immune system, and involves a wide array of proteins that serve as a marker to indicate the presence of a pathogen to enhance particular phagocytic cells (a specialised cell capable of engulfing and absorbing pathogens) such as macrophages (A type of white blood cell that surrounds and kills microorganisms, removes dead cells, and stimulates the action of other immune system cells).
