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Our immune system is a system of biological structures and processes that protects against disease by identifying and killing pathogens and tumor cells. It detects a wide variety of agents, from viruses to parasitic worms, and needs to distinguish them from the organism's own healthy cells and tissues in order to function properly. The immune system is also involved in the recognition and rejection of foreign cells and tissues [which is why organ transplant and blood transfusion have to be carefully managed]. Detection is complicated as pathogens can evolve rapidly, and adapt to avoid the immune system and allow the pathogens to successfully infect their hosts.
The Lines of defence
When we are attacked by a virus or bacteria or other pathogen, the body has various lines of defence.
Mechanical - Its first line of defence is largely ‘mechanical’ for example the skin itself. If the pathogen enters the body, however, we may cough or sneeze in order to mechanically eject pathogens and other irritants from the respiratory tract. We may have a runny nose or watery eyes which serve to flush away the pathogen. And urine is also another flushing mechanism. Mucus may be used to trap the pathogen.
Chemical - Our second line of defence is our own antibiotic system. Breast milk contains antibacterials for example. Our skin [unless we wash it to extinction] secretes antimicrobial barriers. The vagina does and so does semen. The stomach contains both acid and ‘friendly flora’. These friendly flora compete with pathogenic bacteria for food and space and, in some cases, by changing the conditions in their environment, such as the pH. This is why antibiotics need to be used sparingly. Most antibiotics target all bacteria, they are not specific- so they can just as easily kill off our own ‘good bacteria’. Furthermore, they do not affect fungi, which is why oral antibiotics can lead to an “overgrowth” of fungi and cause conditions such as thrush.
The third line of defence is then what is called the ‘innate immune system’.
Innate immune system
Microorganisms or toxins that get past the primary and secondary defences and thus successfully enter us will next encounter the mechanisms of the innate immune system.
The innate response is usually triggered when microbes are identified by pattern recognition receptors, these recognise whether the pathogen is a friend or foe. The same response is triggered when damaged, injured or stressed cells send out alarm signals, many of which (but not all) are recognized by the same receptors as those that recognize pathogens. In effect, the defense system recognises ‘the walls have been breached’ and sends out an alarm – whether the external walls have been breached or whether an intruder has breached the security system and has entered in a different way.
There are two types of innate immune system – the non specific immune system and the specific immune system.
The non specific system
These innate immune defenses are non-specific, meaning these systems respond to pathogens in a generic way. They can’t tell what sort of a foe it is, only that it is a foe. ‘Comrades, this one is not one of us!!’
The non specific immune system is present from birth. It is a quick response system effective against a wide range of pathogens and foreign substances. It always gives the same type of response. For example when the skin is irritated by a foreign substance such as poison from a stinging nettle, there is inflammation.
We tend to think of inflammation as a bad thing, but inflammation is a sign that our body is working for us. The symptoms of inflammation are redness, swelling, heat, and pain which are caused by increased blood flow into a tissue. Injured and infected cells release various chemicals that produce fever and the dilation of blood vessels. Both have their uses, as dilation of blood vessels helps the ‘troops’ to get round the system faster to defend the body and heat is often a killer for pathogens.
Mast cells reside in connective tissues and mucous membranes, and regulate the inflammatory response. Mast cells are especially numerous at sites of potential injury - the nose, mouth, and feet, internal body surfaces, and blood vessels. Mast cells use histamine.
Basophils are a category of white blood cells characterized by the presence of granules in their cytoplasm. There aren’t that many, they represent about 0.01% to 0.3% of circulating white blood cells. Basophils can be recruited out of the blood into a tissue when needed. They secrete chemical mediators that are involved in defending against parasites, but they too use histamine.
Use an antihistamine on the common cold and you are actually stopping your own system from fighting back. It is more than likely the cold will take weeks rather than days to get better and you run the risk of it worsening.
The specific immune system
The specific immune system is the one that attacks, disarms and destroys foreign bodies. It responds relatively slowly and is effective only against specific pathogens, however, the response is speeded up on repeated infection with the same pathogen – a phenomenon known as immunological memory.
Every cell has complex molecules on its surface membrane which act as recognition devices. These molecules are called ‘antigens’. The specific immune system uses antigens to determine the type of pathogen and the type of response needed.
In effect, as part of this more complex immune response, the human immune system adapts over time to recognize specific pathogens more efficiently. This adaptation process is referred to as "adaptive immunity" or "acquired immunity" and creates immunological memory. Immunological memory, created from a primary response to a specific pathogen, provides an enhanced response to secondary encounters with that same, specific pathogen. This process of acquired immunity is the basis of vaccination. Primary response can take 2 days and up to 2 weeks to develop. After the body gains immunity towards a certain pathogen, when infection by that pathogen occurs again, the immune response is called the secondary response.
We saw that one of the first responses is that of inflammation, but the injured cells also send out various chemical messengers which enable the cells being attacked or which have been damaged to ask for help and the right sort of help:
- leukotrienes - attract certain white blood cells (leukocytes) that are key to defenses – storm troopers. They are not the only defense, but they are a major part of our defences
- cytokines - include interleukins that are responsible for communication between white blood cells and interferons that are anti-viral – these are key in fighting viruses.
These chemicals recruit immune cells to the site of infection and promote healing of any damaged tissue following the removal of pathogens.
There are then essentially two ways in which the pathogen is specifically attacked – one is called the humoral response and works via antibodies.The other is called the cell mediated response and works via T cells.
Antibody – An antibody is a type of protein that reacts with a specific antigen. There are several types of reaction which tend to depend on the ability of an antibody to bind to the antigen – binding only occurs when the antigen and antibody have complementay shapes. Antibodies are produced by lymphocytes in the bone marrow.This system is sometimes known asthe ‘complement system’ . It is named for its ability to “complement” the other sytems by killing pathogens using antibodies. The pathogen is first marked as foreign – tagged on its surface. This results in the production of peptides that attract immune cells that coat the surface of the pathogen marking it for destruction. This deposition can also kill cells directly by disrupting their plasma membrane.
T cells – The cell mediated response uses cells rather than specific antibodies to attack the pathogens and these cells are lymphocytes called T cells [thymus cells]. These then activat Leukocytes (white blood cells) which act like independent, single-celled organisms and are of various types – each with their own killing role:
- phagocytes (macrophages, neutrophils, and dendritic cells) - engulf, or eat, pathogens or particles. Phagocytes generally patrol the body searching for pathogens, but can be called to specific locations by cytokines.
· Neutrophils travel throughout the body in pursuit of invading pathogens. Neutrophils are normally found in the bloodstream and are the most abundant type of phagocyte, normally representing 50% to 60% of the total circulating leukocytes. During the acute phase of inflammation, particularly as a result of bacterial infection, neutrophils migrate toward the site of inflammation, and are usually the first cells to arrive at the scene of infection.
·Macrophages reside within tissues and act as scavengers, ridding the body of worn-out cells and other debris.
· Dendritic cells (DC) are phagocytes in tissues that are in contact with the external environment; therefore, they are located mainly in the skin, nose, lungs, stomach, and intestines. Dendritic cells serve as a link between the bodily tissues and the innate and adaptive immune systems.
- Natural killer cells - Natural killer (NK cells) cells are leukocytes that attack and destroy tumor cells, or cells that have been infected by viruses. They are also able to attack large pathogens such as parasites. Killer T cells demolish their opponents by punching holes through their cell surface membranes so that the cell contents spill out!
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