How we fight infections

Our 3 lines of defense

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First line of defense Our Epithelia or skin

Our body fights off germs and infections in several ways. The first defences our bodies have against germs are our epithelia. Our epithelia is our skin and the lining of the insides of our bodies, such as those that line our throat, intestines and nose. This epithelia provides a physical barrier through which germs must either cross through or break through, in order to cause infection. The epithelia lining the insides of our body release mucus. Mucus prevents germs from attaching to the epithelia and stops them from crossing. Germs trapped inside the mucus are then cleared away by the beating of little hair-like structures on our epithelia called cilia, which clear the germs away. Our epithelia can also release chemical substances that prevent microbial growth, which are called microbicides. Examples of these microbicides include defensins, cathelicidins, histatins and lysozymes. These chemicals help our body clear pathogens.

Second line of defense Our innate immune system

If germs can get past our epithelia, the first line of defence they face is called our “innate immune” system. Our innate immunity is a broad and generalised system for recognising things inside our body that don’t belong there.
The system recognises special patterns that are found on germs such as chemicals that certain types of bacteria have. These patterns are recognised by our innate. When a pathogen has been identified by our innate immunity, many responses are triggered to start fighting off the infection. 

1) One of the first and major reactions is the release of cytokines. The release of cytokines helps to increase blood flow to the area, so that our immune cells can reach the infected site. We feel the release of cytokines as inflammation, redness and swelling on our body. Cytokines help guide and activate our immune cells, which then begin fighting off the pathogens in the area. 

2) Another reaction that can be inside our body is the “complement system”. The complement system works by coating pathogens in special proteins called complement proteins. These proteins bind and coat pathogens which helps guide immune cells to eat up and kill pathogens. The complement system can sometimes even kill pathogens outright, by forming large holes on pathogens which causes them to eventually burst.

3) Finally, one of the most important types of cells in the innate immune system are “dendritic cells”. These cells act like recon, which scout pathogens and collect chemicals from them to prepare our next line of defence.

Third line of Defense - Our adaptive immune system

This next and final line of defense that our body has is one of the most important and specialized forms of immunity we have, called the “adaptive immune” system. If our “innate immune” system are like normal police officers who are trained to do many things but are not particularly good at fighting off tougher criminals, our “adaptive immune” system are like specialized Navy SEALs, that are specifically designed to fight off particular types of pathogens. As our innate immune system is still fighting off pathogens, the dendritic cells of the innate system (Which remember are like recon), collect chemicals from the pathogens we are fighting. These chemicals that our dendritic cells collect are called “antigens” and they bring them to special parts of our bodies. Once inside our lymph nodes, the dendritic cells shows these antigens to cells inside our lymph nodes. These cells inside our lymph nodes are T-cells and B-cells, which are like our bodies very own Navy SEALs team. The dendritic cell shows the chemicals they collected, and look for T-cells and B-cells that are able to recognize the antigen. When a T-cell or a B-cell inside our lymph node recognizes the antigen, it means that the T-cell or B-cell is especially good at recognizing that chemical signal and is thus able to recognize the pathogen in our body and attack it very efficiently. Once a T-cell or B-cell recognizes the antigen, the dendritic cell gives a message to the T-cell or B-cell and starts telling it to make many copies of itself. Once the T-cell or B-cell has made many copies of itself, these copies then go out to attack the pathogens in our body.

T-cells are able to kill pathogens directly, or by calling other immune cells, like macrophages.

B-cells are special in that they can release a special chemical called an “antibody”. Antibodies are special proteins which are designed to recognize specific antigens. These antibodies travel around our body and will recognize antigens inside our body. When an antibody recognizes an antigen, it binds it and coats its, rendering the pathogen trapped inside the coat of antibody rendered useless and not able to do anything. Once the pathogen is coated, the pathogen can then be cleared away.


B cell (Niaid)

T cell (Niaid)

However, even after the infection has been cleared, our adaptive immune system doesn’t go back to how it used to be. Our adaptive system has a type of memory, such that once it fights off an infection, it is able to fight off the same infection again but much faster. This is because after an infection is cleared, the T-cells and B-cells that were active, still remain in our body in large numbers. If the same infection were to enter our body, our body does not need to activate them again and our T-cell and B-cells are already ready to fight off the pathogen. They can do this so fast that we might not even feel sick, even though they are killing pathogens. This is what makes the adaptive immune system very special and the strongest form of defence we have against infections.

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