There have been many great wars over the years, which have resulted in millions of deaths. But did you know that inside your body, there is an ongoing war with billions of casualties? The MRC Lab of Molecular Biology estimates that every day, we breathe in over 500 billion viruses alone. So what is stopping our bodies being overwhelmed by these microscopic monsters?
Well, we have the immune system to thank for that! The immune system is an extremely complex network of cells and proteins that work together to remove foreign bodies like bacteria and viruses.
Breaking into the Body
A pathogen is a microscopic organism that can cause a disease. The body’s first defence mechanism is the innate immune system, which is a general response to all pathogens. There are two parts to this: the physical barriers to entry and the chemical/cellular response. If a pathogen wants to infect a host, the first challenge it must overcome is to find a way to get inside the body. One of the barriers is the skin, which acts as an impermeable surface, and glands on the skin secrete a mildly acidic substance which forms a protective film over the skin. If bacteria are on your eyes, your tear glands secrete lysozymes, which are powerful enzymes that break down the cell wall of bacteria. If bacteria try to enter through the mouth, they meet your tonsils. This organ has specialised cells that can detect the antigens (surface proteins, which are specific to different organisms) of foreign bodies like bacteria. They then inform the T and B cells nearby, which in turn trigger an immune response to wipe them out. If this fails, the stomach acid will soon destroy them. But let’s imagine that you have a cut. The skin has been breached. Bacteria and viruses can now enter your body.
The innate immune system – Part 2
For now, the bacteria remain undetected because they initially pose no threat. This period is known as the incubation period, where the bacteria will grow and multiply. Eventually, their behaviour changes and they become very aggressive. They either produce dangerous toxins that can kill cells, or they crowd around tissues and disrupt their tasks. The first on the scene to deal with the intruders is your non-specific white blood cells (WBC). A type of non-specific WBCs are called macrophages. These cells have small arms (pseudopods ) that encircle and engulf bacteria in a process called phagocytosis. The cell then cleverly displays the antigen of the bacteria on its cell membrane so that other immune cells can identify it. Digestive enzymes break down the now helpless bacteria, and the cell releases its contents. Macrophages continuously move around the bloodstream and your lymph nodes to detect bacteria.
If this step isn’t enough, the macrophages release cytokines ( protein used for cell signalling), almost like a distress signal for help. After this, neutrophils arrive. These are more potent cells that either use phagocytosis or release antimicrobial factors to kill bacteria. However, they are so ‘volatile’ that they are programmed to die (apoptosis) to prevent damage to healthy body cells. But in this process, they release NETs, a web of nucleic acids (a type of molecules that makes up DNA and RNA), and bacterial enzymes. These NETs trap and kill bacteria. But in severe cases, the innate immune response ( a general response to infections) isn’t enough, and now the body must resort to the adaptive immune response (an immune response that is specific to the invader).
The Adaptive Immune System
Now, dendritic cells receive the cytokines from the macrophages and enter the battlefront. These cells can present the antigen of the invader on its cell wall. They are found in the lymph nodes, the spleen or tissues exposed to the external environment, e.g. the skin. With the antigen on it, the cell travels back to the lymph nodes. Here, they try to find lymphocytes (immune cells), whose receptors are complementary to that of the foreign antigen ( both fit together like a jigsaw). Once a match is found, the lymphocytes become ‘activated’, and they start to divide rapidly. The two types are B and T lymphocytes. T cells occur in different forms, and each form has a different role. For example, there are helper T cells which aid the maturation of B cells from memory B cells. There are also the killer T cells that cause infected cells, and tumour cells, to burst in a process called lysis (when the cell wall or membrane ruptures, causing the cell to die). Finally, there are memory T-Cells, which rapidly divide when the same bacteria tries to invade again. On the other hand, B-cells are involved in secreting antibodies, which are complementary to the antigen of the bacteria. They bind to the antigen, and from here, a few things can occur: they can prevent the toxin from taking effect, they can act as markers, which macrophages can quickly identify and destroy or they can activate the complement system.
The Power of Proteins
The complement system is another complicated process involving proteins. These are usually inactive and roam around the bloodstream. During activation (e.g. by an antibody), three different types of reactions could take place, but each of them will form a protein called C3 on the surface of the bacteria. The C3, in turn, activates other complement proteins which bind to the C3 to form the protein C5. Now, the C5 causes another cascade of reactions to occur, ending in a protein complex known as C5b-9, or the Membrane-Attack-Complex. This complex essentially creates pores in the cell wall of the bacterium, causing the bacterial cell to die via lysis.
Vanquishing the Virus
So now we know how the body deals with bacteria, but what about viruses? These ‘organisms’ (depends if you believe that they are alive) contain genetic material, DNA or RNA, enclosed by a protein layer called a capsid. A virus binds to the surface receptor of a cell and tricks the cell into believing it is something useful, and so it enters the cell by a process called endocytosis. Now, once inside the cell, the virus makes its way to the nucleus, where it releases its genetic material. The cell reads this new information and starts to produce more viruses inside. Then, the virus causes the cell to burst via lysis, releasing more viruses to infect more cells.
You might think that when a virus is inside the cell, it can’t be detected. However, this isn’t exactly true because your cells have a unique system to deal with this. Essentially, they have to display fragments of their internal proteins on their surface. If a virus is in a cell, the protein fragments will end up on the surface of the cell. Natural killer T cells and cytotoxic T cells detect the foreign protein and release cytotoxic ( literally meaning toxic to cells) factors that kill the cells via apoptosis, and prevent them from spreading.
Also, just like with bacteria, antibodies can bind to a virus floating in the blood and makes it easy for the macrophage to ‘see’ them and engulf them. Alternatively, the antibodies can activate the complement system, which will destroy the capsid of the virus. Another interesting defence mechanism is the use of proteins called interferons. They inhibit the virus from multiplying, so manage the risk of the infection spreading.
Keeping in mind that this article is a simple summary, it is evident that the immune system is extremely complicated but very fascinating. The interesting thing is that people can use their own immunity for a disease to create herd immunity. Not everyone can get a vaccine, e.g. if they have immunodeficiency disorders. Still, if most others have been immunised, it becomes tough for a disease to enter a community, thus keeping them safe. The immune system is one of the many intriguing parts of our incredible body!
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- Dr A Mandal. 2013, What are Dendritic Cells?.