Nowadays, we all must be aware of the term Immune System because of the pandemic situation of COVID-\(19\). It has become very important for all of us to understand how to keep our immune system healthy because this is the main system in our body that helps us to fight against diseases. The immune system also plays a very important role in allergies, autoimmune diseases, and organ transplantation.

The ability of the body to protect itself against disease-causing organisms is known as immunity. Every day, our bodies come into contact with a variety of pathogens, although only a small percentage of these cause disease. The reason for this is that our bodies have the ability to produce antibodies against certain viruses, which protects us from sickness. Immunity is the name given to this defence mechanism. Continue reading to know more.

Immune System Definition

The immune system is defined as a unique and complex network of cells, organs, and proteins that protects our body from multiple types of diseases and infections. It recognises and responds to the foreign antigens, and also remembers them so that if in the future the same person is attacked by the same pathogen, then the immune system can recognise and destroy the antigen or pathogen quickly.

Immune System Diagram

Following is the diagram of the organs in the human immune system:

Fig: Human Immune System

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Immune System Organs

The human immune system involves lymphoid organs, tissues, cells, and antibodies.

1. Lymphoid Organs: Lymphoid Organs are those organs where the maturation and proliferation of lymphocytes take place. There are three types of lymphoid organs:
   1. Primary lymphoid organs
   2. Secondary lymphoid organs
   3. Tertiary lymphoid organs

1. Primary Lymphoid Organs

a. Primary lymphoid organs are those organs where B and T-lymphocytes mature and acquire antigen-specific receptors.
b. These organs are the sites of origin and proliferation of lymphocytes.
c. After the maturation of lymphocytes, they migrate to the secondary lymphoid organs.
d. Primary lymphoid organs include bone marrow and thymus.
i). Bone marrow: It is the main lymphoid organ where all blood cells, including lymphocytes, are formed. Maturation of B-lymphocytes occurs here only.

Fig: Bone Anatomy

ii). Thymus: It is the site where T-lymphocytes mature. It is situated near the heart and beneath the breast bone, which keeps reducing with age.

Fig: Structure of the Thymus

2. Secondary Lymphoid Organs

a. Secondary lymphoid organs are the sites where B and T-lymphocytes interact with the antigen and then proliferate and differentiate to become effector cells.
b. These organs include lymph nodes, spleen, tonsils, Peyer’s patches of the small intestine, and Mucosal associated lymphoid tissues (MALT).

i) Lymph Nodes: These are small solid structures found at intervals along the lymphatic system composed of lymphoid tissue. These are the first organised lymphoid structures that encounter the antigens entering the tissue spaces. They act as filters for the lymph, preventing foreign particles from entering the bloodstream. It also produces lymphocytes and plasma cells.

ii) Spleen: It is a large bean-shaped organ beneath the stomach. It is the largest single mass of lymphoid tissue in the body. In the foetus, the spleen produces all types of blood cells, but in adults, it only produces lymphocytes and phagocytes. It is a large reservoir of erythrocytes. Spleen is known to trap and respond to blood-borne antigens. It is not supplied with the lymphatic vessels, unlike the lymph nodes.

iii) Tonsils: There are usually six tonsils that act as filters to protect the body from bacteria and viruses. It also helps in the formation of white blood cells.

Fig: Types of Tonsils

iv) Peyer’s Patches: These are clusters of lymph nodes found in the small intestine, especially along the ileum. They produce lymphocytes.

Fig: Peyer’s Patches

v) Mucosal-Associated Lymphoid Tissues (MALT): MALT aggregates lymphoid tissues located within the lining of the major tracts like respiratory, digestive and urinogenital tracts. It constitutes about \(50\% \) of the lymphoid tissues in the human body. They are the centres of lymphocyte production. Apart from lymphocytes, phagocytic macrophages and dendritic cells are also present.

Fig: Mucosal-Associated Lymphoid Tissues (MALT)

3. Tertiary Lymphoid Organs

a. Tertiary lymphoid organs usually contain very few numbers lymphocytes that assume their role when they encounter the antigens that cause inflammation.
b. These organs also play a prominent role in the immune response to cancer.

Immune System Cells

The different types of cells involved in the immune system include the white blood cells (WBCs) or leukocytes that circulate in the body in blood vessels and lymphatic vessels. They form the second line of defence and work when the first line of defence is broken. There are two types of WBCs:
1. Phagocytes: These cells engulf the pathogens by phagocytosis, which are of the following types:
a. Neutrophils: These are the most common type of phagocytes that ingest and kill bacteria, and also release certain chemicals that kill other bacteria nearby.
b. Monocytes: These cells are liberated at the site of infection, where they get differentiated into macrophages.
c. Macrophages: These are the large phagocytic cells that develop from monocytes. They wander about in the interstitial fluid, eating any bacteria or virus-infected cells and killing the microbes by phagocytosis.

2 .Lymphocytes: These are the type of leukocytes that helps the body to remember the previous invaders and recognize them if they come back and attack again. These cells are of two kinds:
a. B-lymphocytes: These cells produce antibodies and send a signal to the T-lymphocytes to be alert.
b. T-lymphocytes: These cells help the B-lymphocytes to produce antibodies and attack the antigens by themselves.
c. Natural Killer Cells (NK cells): These cells do not attack the invading microbes directly. They kill the infected cells of the body by releasing a protein called perforin that forms a hole in the plasma membrane of the virus-infected cell or cancer cell.

Fig: Types of Immune System

Functions of Immune System

The functions of the human immune system are as follows:

1. It protects our body from foreign particles like bacteria, viruses, fungi, and toxins.
2. It consists of  B-lymphocytes that produce antibodies to fight against any antigen or pathogen that may have entered the body.
3. It consists of T-lymphocytes that promote antibody production by B-lymphocytes and kill the compromised cells.
4. It also consists of NK cells that secrete perforins which attack the cancerous cells.
5. It has the ability to recognise and neutralise a vast variety of foreign molecules or antigens.
6. It contains memory cells that provide immediate protection when the body of a person is attacked by the same antigen or pathogen for the second time.

Disorders of Immune System

Following are the disorders of the immune system:

1. Allergies

1. Allergy is the hypersensitiveness of a person to some foreign molecules coming in contact with or entering the body.
2. The substances that cause allergic reactions are called allergens. E.g., dust, pollen, mould, spores, bacteria, foods, heat, etc.
3. Allergy mostly affects the skin and mucous membranes. Symptoms include sneezing, watery eyes, running nose, and difficulty in breathing.
4. The allergic tendency is genetically passed from the parent to the child.
5. It is characterised by the presence of large quantities of IgE antibodies (sensitising antibodies) in the blood that combines with the receptors present on the mast cells and basophils in the body.
6. Histamines play a critical role in allergic reactions, which cause inflammatory responses.

7. Some forms of allergy are as follows:

Forms of allergy	Cause of allergy	Symptoms
Hay Fever	Caused due to the pollen of grasses, trees, and other plants. This results in the inflammation of the membrane lining the nose and sometimes of the conjunctiva also.	Sneezing, running or blocked nose, watering eyes.
Asthma	Caused due to the swelling up of the tissues surrounding the bronchioles of the lungs, due to which the bronchioles get compressed.	Difficulty in breathing.
Anaphylaxis (or Anaphylactic shock)	Caused after getting the injection of an antigen in a few minutes that affects all the tissues of the body.	Drastic fall in the blood pressure, unconsciousness, and the individual may die within a short time.

Fig: Allergy Symptoms

2. Autoimmune Diseases

1. If the immune system fails to recognise ‘self’ from ‘non-self’ and starts destroying the body’s proteins, it leads to some malfunctions called autoimmune diseases.
2. Autoimmune diseases mostly happen when certain genes start showing abnormalities like those of antibodies, T-cell receptors, and MHC genes (major histocompatibility complex genes).
3. Autoimmune disorders occur more in women than in men and seem to run in some families.
4. The environment also plays a role in the induction of autoimmune diseases.
5. These diseases are also caused because of some drugs, chemicals, pesticides, and toxins.
6. An increase in the number of helper T-cells and a decrease in the number of suppressor T-cells are the causes of autoimmune disorders.
7. These diseases are caused by self-reacting antibodies.
8. Autoimmune diseases depend on which type of self-antigen is involved. For example, if the self-antigen is RBC, then it destroys its own RBC causing severe anaemia; if the self-antigen is a muscle cell, it destroys the muscle causing severe weakness (a condition called myasthenia gravis) and if the self-antigen is liver cells, it causes damage to its liver cells resulting into a chronic case of hepatitis.
9. Other examples of autoimmune diseases include Hashimoto’s thyroiditis, systemic lupus erythematosus, and rheumatoid arthritis, SCID, juvenile diabetes type-I, etc.
10. Autoimmunity is developed either by infection of pathogens, particularly viruses, or deficiency of certain proteins.

3. Immunodeficiency Diseases

1. Immunodeficiency diseases are caused due to the weakening of our body’s immune system that leads to repeated microbial infections.
2. These diseases are of two types: primary and secondary.
   a. Primary Immunodeficiency Diseases: These diseases exist from birth in which a person may be without B-cells or T-cells or both. For e.g., SCID.
   i. SCID (Severe Combined Immunodeficiency Disease):
   1. A person suffering from this disease lacks both B-cells and T-cells from birth.
   2. It is a serious genetic disease in which the person is highly susceptible to infections.

b. Secondary Immunodeficiency Diseases: These diseases are caused due to a number of factors like malnutrition, infections, metabolic disorders, malignancy, and cytotoxic drugs. E.g., AIDS and Hodgkin’s disease.
i. AIDS (Acquired Immunodeficiency Syndrome)
1. It is a disorder of the cell-mediated immune system of the body in which there is a reduction in the number of T-cells that stimulate B-cells for antibody production.
2. This results in the loss of natural defence against viral infection.

ii. Hodgkin’s Disease
1. It is a malignant disease of lymphatic tissue that helps the immune system to get rid of the waste and fight against the diseases.
2. This disease is a type of lymphoma, a blood cancer that starts in the lymphatic system.

Summary

Thus, we conclude that our body has various mechanisms for protection from foreign invaders and preventing infection. These protective mechanisms in the body form the immune system that provides immunity against invading pathogens. The unique property of the immune system is that it never attacks the body’s proteins but always destroys the foreign proteins. Now, during this COVID-\(19\) pandemic situation, we must keep our immunity strong so that the virus cannot attack and enter our bodies. So, we should take precautions like always wearing masks, cleaning our hands frequently, maintaining a safe distance, and getting ourselves vaccinated as soon as possible.

Frequently Asked Questions

Q.1. What are the signs of a weak immune system?
Ans: The signs of a weak immune system are:
a. Always prone to cold, cough, and fever.
b. Facing lots of stomach problems like gas, constipation, and diarrhoea.
c. Slow healing of wounds.
d. Getting tired very soon.

Q.2. What are the signs of a good immune system?
Ans: The signs of a good immune system are:
a. Not getting stressed very easily.
b. If got an injury, it gets healed up quickly.
c. Not falling ill frequently like having a cold, cough, or fever.
d. After doing strenuous work also, not getting tired.

Q.3. What foods are bad for the immune system?
Ans: Foods bad for the immune system includes the following:
a. Foods containing high salt and sugar content.
b. Fast foods like noodles, Manchurian, etc.
c. Fried foods.
d. Foods containing high omega-6 fats content.
e. Artificially sweetened foods and beverages.

Q.4. How can I strengthen my immune system?
Ans: I can strengthen my immune system in the following ways:
a. Eating a balanced diet.
b. Doing regular exercise.
c. Washing hands always, i.e., before and after taking food.
d. Getting enough sleep.
e. Trying to minimize the stress.

Q.5. What can damage your immune system?
Ans: Our immune system can be damaged by taking alcohol, smoking, poor nutrition, getting stressed, always taking medicines, not getting proper sleep, etc.

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