How do our bodies fight disease?

12 min readJul 17, 2020

While learning about Machine Learning use cases in drug discovery and vaccines, I stumbled upon the various aspects related to Immunity. I am fascinated by the beauty of the various mechanisms in our bodies. I thought it is necessary for all of us to know how our body responds to disease. It also helps us appreciate the Scientists and doctors who are working around the clock to fight the pandemic of this generation — Covid19.

Let us start this fascinating journey into biology and find answers that are often arising in these testing times of the Covid19 pandemic when so many biological terms are quite often quoted in the news media.

What Causes Infections?

Infections and disease-causing agents are called pathogens such as Bacteria, Virus, Fungi, and Protozoa. Our body must stop these pathogens which can come from a lot of different places. We get in contact with them through touching skin or surfaces, having sex, and breathing in drops from someone else’s sneeze or cough, etc., They can travel through the blood that comes from a shared needle or an insect bite. We can also get germs from contaminated food or water.

https://www.ck12.org/book/ck-12-biology-concepts/r5/section/13.48/

What is the Immune system (IS)?

The IS is a network of organs, tissues, and cells, resembling a well-fortified castle. Just like the outside of the castle was protected by a moat and high stone walls, the body is protection from Physical barriers. Inside the castle, soldiers were ready to fight off any invaders that managed to get through the outer defences. Similarly, the immune system triggers the release of special cells. These cells travel to where the trouble is, attack the intruder, and fight. Only pathogens that can get through all lines of defence can harm the body. Immunity is the ability of the body to fight a pathogen at all the lines of defences.

The IS system protects our body from infection with layered defences of increasing specificity. We can classify the IS into two categories Innate Immunity and Adaptive Immunity.

Innate immunity is what we are born with. It is the ability of our body to defend itself against any pathogen in general. This immunity comes from those barrier body parts as well as some specialised cells. The innate IS provides an immediate, but non-specific response.

If pathogens successfully evade the innate response, the next level of protection is the adaptive IS which is activated by the innate response. The IS adapts its response during an infection to improve its recognition of the pathogen. It is a specific response.

This improved response is then retained after the pathogen has been eliminated, in the form of an immunological memory, and allows the adaptive immune system to mount faster and stronger attacks each time this pathogen is encountered.

Barriers of Prevention

The first line of defence consists of physical barriers that keep most pathogens from entering our body.

The skin: This is the most important barrier as its outer layer is very difficult for pathogens to penetrate.

Mucous membranes: They provide a mechanical barrier at body openings. They also line the respiratory, Gastro-Intestinal (GI), urinary, and reproductive tracts. Mucous membranes secrete mucus, a slimy substance that traps pathogens. The membranes also have hair-like cilia. The cilia sweep mucus and pathogens toward body openings where they can be removed from the body.

Sweat, mucus, tears, and saliva all contain enzymes that kill pathogens. Urine is too acidic for many pathogens, and semen contains zinc, which most pathogens cannot tolerate. In addition, stomach acid kills pathogens that enter the GI tract in food or water.

When you sneeze or cough, pathogens are removed from the nose and throat. In case of the corona virus, the spread could be through the air or the virus can persist on the surface. This is the reason why WHO/C DC recommends we wash hands with soaps, avoid touching surfaces, wearing masks, and maintain social distancing. The methods are preventive and impede the virus to spread.

If the pathogen has already broken past the first line of defence, it manifests in symptoms such as headache, diarrhoea, vomiting, fever, coughing, runny nose, etc., These symptoms indicate that a person is infected.

As you are aware, why different test are done for detecting Covid19 for people in different stages. Swab tests for asymptomatic patients (since the presence of Corona virus is likely to be in the nasal passages) and anti-body tests for symptomatic patients.

The Lymphatic System

We must understand some details about the mechanisms of IS to develop an appreciation for the work of scientists and immunologists who are working hard, trying to figure out Therapeutics and Vaccines for Covid19.

The immune response mainly involves the lymphatic system; which is a major part of the IS. It is a network of fine tubes throughout your body that collects fluid called lymph from tissues — its function is to pick up dead cells and germs. The waste is filtered out at small bean-shaped lymph nodes, and the liquid goes back into your bloodstream. An infection can make the nodes swell; you may have felt them in your neck when you had a sore throat or cough. The following is the picture of the Organs that constitutes Lymphatic System.

The organs of the IS produces leukocytes called lymphocytes. Lymphocytes are the key cells involved in the immune response. Different types of cells are Neutrophils, mast cells, basophils, dendritic cells, Eosinophils, monocytes macrophages and the natural killer (NK) cells. Which we will discuss a little later.

Bone marrow: The soft, fatty stuff that lives inside the bones. Bone marrow makes blood cells, including the various White Blood Cells (WBC) that fight off germs. Types of WBCs are granulocytes (neutrophils, eosinophils, and basophils), monocytes, and lymphocytes.

The human body has as many as two trillion lymphocytes, and lymphocytes make up about 25% of all leukocytes. Most lymphocytes are found in the lymphatic system, where they are most likely to encounter pathogens. The rest are found in the blood. All the cells are type of white blood cells (WBC) also known as leukocytes (leuko means white, cyte means cell). Neutrophils are the most abundant immune cells making up about 60 to 70% of the total population of immune cells.

Macrophages grow from WBCs called monocytes, but they work in tissues, not your blood.

Eosinophils mainly attach to parasites that are too big to ingest in order to kill them.

Basophils and Mast Cells are involved with allergic reactions. Basophils are in the blood; mast cells are in tissues. When these cells find certain antigens, they release histamine to bring immune cells to the area.

Cytotoxic T-Cells and Natural Killer Cells: These cells are two distinct lineages of immune cells that play an important role in the control of infection and in the detection and removal of cancerous cells. Cytotoxic T cells destroy virus-infected cells and some cancer cells. Once activated, a Cytotoxic T cell divides rapidly and produces an “army” of cells identical to itself. These cells travel throughout the body “searching” for more cells to destroy. The T cell releases toxins that form pores in the membrane of the infected cell. This causes the cell to burst, destroying both the cell and the viruses inside it.

Thymus: Located in the upper chest behind the breastbone. It stores and matures lymphocytes. There are two major types of lymphocytes, called B cells and T cells. These cells get their names from the organs in which they mature. B-cells mature in Bone marrow, and T-Cells mature in the Thymus.

Spleen: Located in the upper abdomen. It filters pathogens and worn-out red blood cells from the blood, and then lymphocytes in the spleen destroy them.

Tonsils and Adenoids: Located on either side of the pharynx in the throat. The adenoids and tonsils work by trapping germs coming in through the mouth and nose. The spleen, tonsils, adenoids, appendix, and small Peyer patches in your intestine are where mature T cells are stored and are often considered as Secondary Lymph Organs.

Lymphatic Vessels and Lymph: Lymphatic vessels make up a body-wide circulatory system. The fluid they circulate is lymph. Lymph is a fluid that leaks out of capillaries into spaces between cells. As the lymph accumulates between cells, it diffuses into tiny lymphatic vessels. The lymph then moves through the lymphatic system from smaller to larger vessels. It finally drains back into the bloodstream in the chest. As lymph passes through the lymphatic vessels, pathogens are filtered out at small structures called lymph nodes. The filtered pathogens are destroyed by lymphocytes.

Functioning of the Lymphatic System

There are 2 types of Immune Responses:

Inflammatory Response: If you have a cut on your hand, the break in the skin provides a way for pathogens to enter the body and infect the wound. The cut may become red, warm, and swollen. This is when the IS activates inflammatory response triggered by chemicals called cytokines and histamines — the first reaction of the body to tissue damage or infection.

The Immune Response: The pathogens will contend with our body’s “immune response”; as the next defense. There are two scenarios in this. The first is, the IS “knows” this pathogen (this defense is faster and immediate) and the second is the pathogen is new (here, the IS will have to work out a way to fight the disease).

Phagocytosis (Phago means eating and cytosis means by a cell) is a process of neutrophils or a type of phagocytes that detect and eat the pathogen. Within the cell, the neutrophil release various enzymes that kill and digest the pathogen. They normally found in the bloodstream and are short lived. They are highly mobile and can enter parts of tissue where other cells or molecules cannot. Neutrophils are usually the first cells to respond to any infection or inflammation. This is due to their heightened ability to sense was going around the body by detecting special chemical signals released by pathogens and by our own body cells that tell them what going around in the body.

Antigens are substances (usually proteins) on the surface of cells, viruses, fungi, or bacteria. Non-living substances such as toxins, chemicals, drugs, and foreign particles (such as a splinter) can also be antigens. Antigens are recognized as foreign to the body by the IS.

Antigens are also found on cancer cells and the cells of transplanted organs. They trigger the IS to react against the cells that carry them. This is the reason why a transplanted organ may be rejected by the recipient’s IS. B and T cells recognize and respond to antigens on pathogens.

There are two types of immune responses: humoral and cell-mediated.

Humoral Immune Response

The humoral immune response involves mainly B-Cells and takes place in blood and lymph. B-Cells must be activated by an antigen before they can fight pathogens. First, a B-Cell encounters its matching antigen and engulfs it. The B-Cell then displays fragments of the antigen on its surface. This attracts a helper T-Cell. The helper T-Cell binds to the B cell at the antigen site and releases cytokines that “tell” the B cell to develop into a plasma cell.

https://www.ck12.org/book/ck-12-biology-concepts/r5/section/13.51/

Plasma Cells and Antibody Production: Plasma cells are activated B cells that secrete antibodies. Antibodies are large, Y-shaped proteins that recognize and bind to antigens. Plasma cells are like antibody factories, producing many copies of a single type of antibody. The antibodies travel throughout the body in blood and lymph. Each antibody binds to just one kind of antigen. When it does, it forms an antigen-antibody complex. The complex flags the antigen-bearing cell for destruction by phagocytosis.

Cell-Mediated Immune Response

The cell-mediated immune response involves mainly T-Cells. It leads to the destruction of cells that are infected with viruses and some cancer cells. All types of T-Cells must be activated by an antigen before they can fight an infection or cancer. It begins when a B-Cell or nonspecific leukocyte engulfs a virus and displays its antigens. When the T-Cell encounters the matching antigen on a leukocyte, it becomes activated.

Helper T-Cells: Helper T cells are like the “managers” of the immune response. They secrete cytokines, which activate or control the activities of other lymphocytes.

Regulatory T Cells: These cells are responsible for ending the cell-mediated immune response after an infection has been curbed. They also suppress T cells that mistakenly react against self-antigens.

Memory Cells : Most plasma cells live for just a few days, but some of them live much longer. They may even survive for the lifetime of the individual. Long-living plasma cells are called memory cells. They retain a “memory” of a specific pathogen long after an infection is over. They help launch a rapid response against the pathogen if it invades the body again in the future.

Most helper T-Cells, Cytotoxic T-Cells die out once a pathogen has been cleared from the body, but a few remain as memory cells. These memory cells are ready to produce large numbers of antigen-specific T-cells like themselves if they are exposed to the same antigen in the future to mount a rapid immune response

Cytokines : Different kinds of cells can make these messengers. Some cytokines trigger and focus on the immune response. They might tell white blood cells where to go or how to destroy a particular germ. One type, interferons, can slow or stop a virus from making copies of itself. Cytokines also tell your body to shut it down after a threat is gone.

Types of Immunity?

Memory B and T cells help protect the body from re-infection by pathogens that infected the body in the past. Being able to resist a pathogen in this way is called immunity. Immunity can be active or passive.

Active Immunity : Active immunity results when an immune response to a pathogen produces memory cells. As long as the memory cells survive, the pathogen will be unable to cause a serious infection in the body. Some memory cells last for a lifetime and confer permanent immunity.

Active immunity can also result from immunisation. Immunisation is the deliberate exposure of a person to a pathogen in order to provoke an immune response and the formation of memory cells specific to that pathogen. The pathogen is often injected. However, only part of a pathogen, a weakened form of the pathogen, or a dead pathogen is typically used. This causes an immune response without making the immunised person sick. This is how you most likely became immune to measles, mumps, and chickenpox.

Passive Immunity : Passive immunity results when antibodies are transferred to a person who has never been exposed to the pathogen. Passive immunity lasts only as long as the antibodies survive in body fluids. This is usually between a few days and a few months. Passive immunity may be acquired by a fetus through its mother’s blood. It may also be acquired by an infant through the mother’s breast milk. Older children and adults can acquire passive immunity through the injection of antibodies.

Immunodeficiency : When the Immune system doesn’t react strongly enough to a pathogen, it is called immunodeficiency disorder, like AIDS. An autoimmune response happens when your body mistakes your tissues or organs for invaders and attacks healthy cells. This deficiency can cause serious illnesses like rheumatoid arthritis, Crohn’s disease, type 1 diabetes, and lupus.

Concluding remarks

Being aware of the functioning of the Immune systems :

Would protect us from dangerous information that is promoted as “miracle” cures, “Immunity boosting” fads, and “quack” medicines from the profiteers on the prowl.
Help us understand and appreciate the tremendous work that is being carried out by our scientific community to defeat pandemics like Corona virus.
Help us understand the information put out by WHO/CDC & NIH in a rational way and educate the community to help fight this deadly Covid19 pandemic.