How does our body fight pathogens?

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The human body has an inbuilt mechanism to produce a response whenever an outside pathogen bearing an antigen enters the body. The antigen is anything that produces antibodies; it could be a protein or carbohydrate etc. Common pathogens that the body needs protection from include viruses, bacteria, fungi and other disease-causing microbes. The mechanism of immune response is quite complex, let’s dig deeper!

Mechanism of immune response

Immunity can be of two types, the first type of immunity is present from birth and is non-specific, this is called innate immunity. The second type of immunity is developed after birth and this immunity is specific to the antigen that is causing the antibody reaction. This immunity is called acquired immunity. Let us learn more about innate and acquired immunity deeply.

Innate Immunity

Innate immunity has two lines of defence:-

  • The first line of defence has a physical barrier and a physiological barrier.
  • The second line of the defence system has a cellular barrier and a cytokinin barrier.
Physical barrier

The physical barrier includes the skin majorly. Skin is like the first line of defence because it prevents the entry of pathogens into our body and hence provides protection.

Physiological barrier

This barrier mainly includes mucus which has lysozyme present in them. Eg: mucus in the saliva. HCl is present in the stomach and due to HCl being extremely acidic it kills a variety of antigens too. Other alkaline secretions in the intestine help in fighting antigens too. The antigens that cross the first line of defence can be trapped by the mucus of the physiological barrier.

Cellular barrier

This barrier is the second line of defence. If the antigen that can cause disease in our body has crossed the first line of defence it goes to the second line of defence. The cellular barrier includes cells like macrophages, monocytes, neutrophils and natural killer cells. These four cells make the major cellular barrier of immunity and produce an immune response against antigens.

Let’s talk about the macrophage cell first. The macrophage cell is the first cell that the antigen will encounter if it crosses the first line of defence. The macrophages are majorly responsible for showing phagocytosis of the antigen by engulfing it. 

Sometimes the macrophage destroys the antigen completely; however, sometimes the antigen can be too strong for the macrophage to handle. If this happens the macrophage cells call upon monocytes and a monocyte-macrophage system is formed also called the reticuloendothelial system.

The pathogen that enters the cell releases a toxin and the macrophages release a protein in response to the toxins and form a substance called pyrogen. Due to the presence of pyrogen in our body the temperature of our body rises which is done by the hypothalamus (the thermoregulatory centre of a body). During this time our body experiences fever. Hence we say that in response to fever antipyretic drugs are given.

mechanism of immune response


The neutrophil cells also show phagocytosis just like the macrophage at the site of infection and produce an immune response towards it.

Natural killer cells or NK cells

They are also included within the cellular barrier and are majorly responsible for destroying cancerous cells. They are like the snipers in our immune system. Whenever the sniper encounters a cancerous cell emerging in any particular place in our body they destroy the cancerous cells. Cancer cells show neoplastic transformation and natural killer cells kill them by secreting perforin, this creates spots inside the cancer cells and cell lysis occurs as the fluid moves inside.

Cytokinin barrier

The cytokinin barrier is mainly responsible for protection from viruses. Virus-infected cells of a body produce a substance called interferons that do not help the cells that have already been affected by viruses but do provide protection to non-viral infected cells by inhibiting the replication of the virus.

Acquired immunity

Acquired immunity is unlike innate immunity. This type of immunity is not present from birth and is developed after birth. This immunity is highly specific and has discriminating power hence can discriminate between self and non-self cells. This immunity can produce a specific reaction to a particular antigen or disease-causing pathogen. This immunity has a diverse and wide range of actions along with a characteristic trait of memory. The antibodies that are created in response to antigens have a memory and they remember the antigen that had entered the cell. 


The first time that the pathogen enters the cell it produces a special mechanism of immune response called primary response which is not very strong however once the immunity has captured the pathogen and memory is created the next time the pathogen attacks our immunity produces a secondary response/ anamnestic response which is very strong.

Macrophage, B cells and dendritic cells are called APC. APC stands for antigen-presenting cell. Whenever the pathogen attacks the body the APC engulfs it as soon as it attacks. The APC after gulping the antigen goes to the nearest lymphoid organ. When the lymphoid organs encounter the engulfed pathogen it produces T helper cells. The T helper cells release interleukin 2. Due to interleukin-2, the B cells that are present in our immune system engulf the pathogen to become hybridoma cells. 

The hybridoma cells later divide into plasma cells and memory cells. The plasma cell produces 2000-20000 antibodies per cell per second. This antibody is used to attack the antigen and to kill it. The memory cell, however, is preserved for the future so that when the antigen attacks the body again the memory of that antigen is already present and more antibodies can be produced that can administer a stronger secondary response. This type of immunity due to antibodies is called Humoral Immunity and the mechanism of immune response is called the humoral immune response. 

mechanism of immune response

B cells and antibodies

The B lymphocytes cells are the B cells that are responsible for producing antibodies which are proteinaceous and are produced in huge amounts into the bloodstream. The antibodies tackle the antigens and kill them so that they do not cause much harm to our bodies. The antibodies have another name called immunoglobulin. The immunoglobulins are a Y shaped structure also called H2L2 that is 2 heavy polypeptide chains and 2 light polypeptide chains. Light chains have fewer amino acids as compared to heavy chains. Disulfide bonds are present between the chains. The antibody recognises the antigen by binding with it with the help of the antigen-binding site and then destroys it. There are five types of immunoglobulins:-

  1. Immunoglobulin G
  2. Immunoglobulin A
  3. Immunoglobulin E
  4. Immunoglobulin M
  5. Immunoglobulin D
Antibodies IgG IgM IgA IgE IgD

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The thymus gland is also a lymphoid organ. The cells of the thymus gland are called thymocytes. The thymocyte cells are of two types CD8 cells and CD4 cells. The CD8 cells are responsible for producing cytotoxic T cells, suppressor T cells and memory T cells. The CD4 cells are responsible for producing T helper cells.

The mechanism of immune response mediated by T-cells is called Cell-mediated immunity (CMI). These cells are responsible for killing antigens and pathogens that are present in cells and tissues and are not directly coming from outside hence cannot be killed with the help of antibodies. 

Acquired immunity can further be divided into two types natural and artificial.

Naturally acquired immunity

It is produced by the body itself and is a time taking process that has no side effects. For example, when the pathogen first enters the body it becomes an antigen and the body shows an antibody response. This is called natural active immunity.  In childbirth, immunoglobulin G and immunoglobulin A are transferred from the mother to the foetus so that the newborn baby can have some sort of immunity; this is also called passive natural immunity.

Artificially acquired immunity

It is not produced by the body itself but is induced. It is fast and produces an instant immune response. Artificial immunity is short-lived.  Active artificial immunity can be that of a vaccine. Whenever a vaccine is injected into the bloodstream the body produces an antibody response and active artificial immunity is generated. In passive artificial acquired immunity preformed antibodies are produced or injected into the individual’s bloodstream for example when a snake bite occurs anti-venom is given which is antibodies to fight off the toxins i.e.venom of the snake.

Vaccinations and the underlying mechanism of immune response

In 1948 in Geneva WHO decided to tackle health problems at the global level. In 1978 WHO launched a health immunisation programme for 6 diseases. Alfred Jenner formed the first vaccine for smallpox.

Vaccines are antibody provoking agents.


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Whenever a vaccine is introduced into the body a mechanism of immune response is activated which leads to the production of antibodies. This process is known as immunisation. The main principle of the vaccine is to generate memory cells and B cells/ B-lymphocytes. This is mainly done so that the body can remember the antigen that will attack in the future and show a strong secondary/ anamnestic response. For vaccines, we usually take attenuated viruses, bacteria or other antigens and inject them into our body so that the primary response can be shown. Sometimes the pathogen which causes the disease can be killed and inserted into the body or sometimes active toxins which are released by those antigens can be injected. 

These days mRNA vaccines are also being created and used which do not produce the antigen in the body they only go and change the genetic structure so that the required proteins can be produced which will show a strong secondary response. Eg- Hepatitis-B vaccine from yeast (recombinant DNA technology)

Allergies and Autoimmune Disorders


They can be defined as hypersensitive reactions of the immune system towards an organism or antigen that is not harmful. Antigens for allergic reaction can be dust, wheat, milk, pollen grains, chocolate, nuts, walnuts etc. 

mechanism of immune response (allergies)

The immunoglobulin E is IgE plays a major role in an allergic reaction. Whenever an allergen that is the antigen that is causing the allergic reaction enters the body it binds with immunoglobulin E. The immunoglobulin E is present on the mast cell and the mast cells start to release histamine, which is a vasodilator and a Bronchial constrictor. 

When histamine is released the blood vessels dilate and the capillaries become more permeable so the blood that is filtered by blood capillaries also increases and low blood pressure occurs. The bronchioles start to constrict and hence breathing is affected too. This leads to breathlessness and sometimes unconsciousness as well. This is also called an anaphylactic shock and if nothing is done it can lead to death as well.

What has to be done?

When an individual has an allergy condition he or she has to carry an EpiPen with himself or herself all the time. In allergic conditions antihistamines, adrenaline and steroids are usually given to patients. Allergies can occur from any type of foreign material and are very common these days. Several individuals suffer from allergies such as asthma or nut allergy or pollen grain allergy. Make sure to carry an Epipen with you always if you have any allergies.

Autoimmune disorders

Autoimmune disorders occur due to the hypersensitive reaction of our immune system. The immune system has cells which are called T helper cells. The helper cells are very important for cell-mediated immunity and protecting us from antigens and foreign organisms.

The T helper cells have a feature that they can differentiate between cells that belong to our own body which can be termed as self cells and cells that do not belong to our body which can be termed as non-self cells. T helper cells have a major histocompatibility complex that is also called MHC. MHC proteins are from MHC genes that are present on chromosome number 6. 

Since T helper cells can differentiate between self and non-self cells they only attack non-self cells which are antigens or foreign pathogens that can cause harm to our body. To prevent T helper cells from attacking self cells, suppressor t cells are also present in our immune system so that the t helper cells do not cause an exaggerated immune response to self cells.

What happens in autoimmune disorders?

In autoimmune disorders, the number of suppressor T cells decreases and hence t helper cells start attacking self cells. The reason for this can be genetic or is usually unknown. Some examples of autoimmune disorders are rheumatoid arthritis, myasthenia gravis, type 1 diabetes, exophthalmic goitre, multiple sclerosis etc.

When an organ transplant occurs the part of the immune system which is responsible is the cell-mediated immune system of the T cells. Since the T helper cell can recognise a self cell from a non-self cell due to MHC proteins, as soon as an organ transplant occurs, that is an organ from a different member of the same species is transplanted, the helper T cell immediately recognises it and attacks it. 

This is the reason that individuals who undergo organ transplants have to take immunosuppressants all their life. Hence individuals that are very close genetically to the individual who requires organ transplant are chosen for organ transplantation. 

How to optimise your immunity

Several companies claim their products improve your immune system. Especially during the covid-19 pandemic a lot of companies came up with products like copper enriched water, blueberry, and certain supplements that can help you optimise and improve your immune system. However, these claims are usually not true. A better way in which you can improve and optimise your immune system is to take a well-balanced diet, exercise regularly, have the required 7 to 9 hours of sleep and by taking vaccines.

Vaccines are like practising for a test for our immune system, when there is time to show an immune response to the antigen. Otherwise, immunity is generally genetic to every individual.

boost immunity

Some parents don’t let their kids play out in the dirt/mud fearing that they may get sick due to bacteria and other antigens. However, this helps the kid to develop immunity as it gives a lot of exposure. Due to kids staying at home all the time,  their immunity is getting too hypersensitive to everything and hence allergies are on the rise in developed areas. Thus, the mechanism of immune response is very complex and a lot of research is yet to be done. 

Let your children play outside and keep reading at MBD for more exciting content.

Team MBD

Read more- Is Vaccination a key to Immunization? (

Immunity – an overview | ScienceDirect Topics

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