11.1 - Body Defence

Do you know that when you are sick like when you are having a fever, headache or rash is because your body is battling with whatever infection that is happening in your body? The fact that we are experiencing these symptoms proves that our immune systems are working to help our body recover. So, how does the immune system work?

Pathogens are recognized by our body as foreign antigens. When a pathogen tries to infect our body cells, it will activate the body's immune system and trigger an immune response. During the immune response, proteins called antibodies are being produced by lymphocytes in order to destroy the antigens. Now that these antigens have been removed from the body, your body is said to have developed an immunity towards the disease. The question now is how do our body defense systems remove these pathogens?

Phrases and Definitions

• Immune system: The immune system is composed of cells, tissues and organs which work together to help the body fight against diseases.
• Pathogens: Disease-causing microorganisms which can be bacteria, viruses, parasites and others.
• Immune response: A biological reaction that happens within the body of an organism in order to destroy pathogens such as antigen-antibody interaction.
• Antigen: A molecule which can be found on the surface of a pathogen that can stimulate an immune response.
• Antibody: A characteristic Y-shaped protein that is produced by lymphocytes to fight off infections. Also known as immunoglobulin (Ig). |
• Lymphocyte: A type of white blood cell which can be B-lymphocyte, T-lymphocyte or natural killer cell.
• Immunity: The ability of an organism to resist a particular disease or infection.

Figure 1 - Antibodies are specific proteins that bind to specific antigens (just like the lock and key hypothesis!) Note the different shapes of the antigen.

There are three lines of body defense in our body to fight against diseases, known as

• The first line of defense
• The second line of defense
• The third line of defense

The first line of defense consists of the physical and chemical barriers that prevent pathogens from entering the body. It is non-specific and generally defends the body against all pathogens. The examples include skin , saliva , tears , mucus , stomach acid , blood clotting mechanism and others.

Figure 2 - Think of the first line of defense as the outermost protective layers of our organs. Tears protect our eyes, saliva in our mouth, mucus that lines the respiratory tract, acidic condition of our stomach and our skin (imagine if you don't have skin, all kinds of microorganisms can infect us very easily).

1. Skin

• The largest organ of the human body that physically prevents microorganisms from entry, because it is tough and difficult for microorganisms to penetrate.
• Constant shedding of the skin cells also prevents microorganisms from having the opportunity to grow.
• Also acts as a chemical barrier due to its ability to secrete sebum. Sebum contains oil and acid which can inhibit the growth of microbes.
• Sweat secretion contains lysozyme which can break down the cell wall of some bacteria.

Figure 3 - There are 3 layers that make up our skin. The outermost layer is the epithelial cells which are keratinized (hard) to prevent microbes from entering.

2. Stomach acid

• Human stomach contains hydrochloric acid which can help destroy bacteria that are present in the food or drinks consumed.

Figure 4 - Stomach acid has a pH of as low as 1.5 to 3.5, which is very acidic. Many of the bacteria and viruses can be killed there.

3. Blood clotting

• The blood clotting mechanism acts quickly to prevent microbes from entering our body through open wounds or injuries.

Figure 5 - Platelets are the cells involved in blood coagulation. This process is very important to prevent excessive blood loss.

4. Mucous membrane

• The mucous membrane that lines the respiratory tract can produce sticky substances called mucus.
• Mucus can trap and prevent irritants such as dust, bacteria or smoke from entering our respiratory systems through the air we breathed in. Lysozyme found in the mucus can also help to kill the bacteria.

Figure 6 - Mucus which can be found lining the respiratory system (mouth, sinuses, throat, lungs) and gastrointestinal tract (stomach, intestines). It is crucially involved in mucociliary clearance (MCC). Mucus in the nose, for example, traps dust particles, microbes, smoke or other irritants. The presence of cilia (hair) sweeps the mucus to the back of the throat to be swallowed or coughed out.

5. Tears, mucus, saliva

• Contain lysozyme , an antimicrobial enzyme which is capable of killing certain types of bacteria.

Figure 7 - Tears do come out of our eyes when our eyes are irritated by dust particles or when cutting onions. Besides keeping our eyes from drying, tears contain antibodies and lysozyme that are able to wash away debris and protect our eyes from infection.

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The second line of defense is there to protect us in the event where the pathogens get past the body's first line of defense. It is also a non-specific defense mechanism. The examples are fever, phagocytosis and inflammatory response.

1. Fever

• Increase in body temperature helps facilitate phagocytic activity and destroys microbes that have entered the body.

Figure 8. Having fever during an illness is our body's natural response to infection. Higher temperature prevents most bacteria and viruses from multiplying and surviving.

2. Phagocytosis

• Phagocytosis is a process of engulfing large particles which can be microorganisms, dead cells or foreign substances present in the body. In layman's terms, it is basically the process of eating.
• The process can be carried out by a type of white blood cells called phagocytes. Examples of phagocytes include neutrophils , monocytes and macrophages.
• The process is simplified as below in Figure 9.

Figure 9 - 1. Phagocyte extends its pseudopodia (in layman's terms, it means 'legs') and engulfs the pathogen. 2. Ingestion of the pathogen later forms a vesicle called phagosome. 3. Lysosome moves to fuse with the phagosome. 4. The result of the fusion is called phagolysosome. Lysozyme from the lysosome is secreted into the phagolysosome. 5. The lysozyme breaks down and kills the pathogen. 6. The remainings of the dead pathogen are excreted out of the cell.

3. Inflammation

• Is an immediate biological response of body tissues to remove irritants such as harmful pathogens, toxins or damaged cells at the early stages of an infection.
• The five cardinal signs of inflammation are pain (dolor), swelling (tumor), redness (rubor), heat (calor) and loss of function (functio laesa).
• Pus is a natural result of the body's inflammatory response. It contains dead white blood cells, microbes and some body tissues.
• The process of inflammatory response is explained in Figure 10.

Figure 10 - 1. Injury to the skin allows pathogens to enter. The injured tissues release histamine which triggers an inflammatory response. 2. Histamine stimulates vasodilation and increases vascular permeability. Increased in blood flow facilitates the arrival of platelets, blood clotting factors and phagocytes to the injured area. 3. Phagocytes carry out phagocytosis to clear away the pathogens. At the same time, the blood clotting process also begins.

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The third line of defense , also called the immune response, will be activated when the second line of defense is breached. It is a specific adaptive response, which means it targets specific pathogens. It depends on the actions of lymphocytes and acts throughout the whole body.

Figure 11 - Lymphocytes produce specific antibodies in response to specific antigens (which is why it is called a specific response).