Concept of immunity


Immunity is the ability of the body to defend against foreign bodies and microbes. Every individual has a certain level of immunity that helps in preventing the invasion of disease-causing agents. Immunity depends on the age,  health status, nutrition, cultural and social practices of an individual. Some people experience frequent infections despite they are protected from immunity. This is because immunity is variable depending upon many factors. One may feel safe in the dusty weather but the other individual fall sick hence each individual`s immune status varies. "Immunity is defined as the ability of the body to fight against invading organisms and disease-causing agents". There are two types of immunity; innate immunity, and adaptive immunity. Innate immunity is a natural defence mechanism present in an individual by birth itself. In a healthy individual, the body has the natural capacity to fight against the invading microorganisms, but in some cases( immunocompromised patients), this may not be the case always. The specific mediators of innate immunity are the body`s physical,  physiological, cellular, cytokine and fluid barriers present in the body.  Acquired immunity is an artificially induced immunity through external agents such as vaccines. Vaccines are the substances that trigger the body`s natural defence mechanism through antibodies. Some of the vaccines are BCG, DPT, DT, TT, MMR, oral polio vaccine, Hepatitis vaccine and cholera vaccine.


Table Of Content

1. Introduction

2. Natural Immunity or innate immunity

3. Acquired Immunity

4. Vaccination and Immunization

5. Immune system

6. Lymphoid organs and their role in immunity




Natural Immunity or innate immunity

1. Physical barriers: Some of the physical barriers that play a vital role in safeguarding the immunity are our skin, mucous membrane, and hair. These barriers when they come in contact directly with the microorganisms will fight them to safeguard us. Epidermis present on the skin is very thick and strong that does not allow the foreign organisms to invade into our body easily. Similarly, the mucus membrane has the ability to produce thick secretions that will trap the dust and microorganisms.

2. Cellular barriers: The major cellular barriers that help in the immune mechanisms are WBCs, lymphocytes, polymorpho-nuclear leukocytes or neutrophils and monocytes. These cells will help to engulf the invading microorganisms.

3. Cytokine barriers: Here the immunity is mediated by interferons - a protein secreted by cytokines. Interferons become active when an antigen enters the bloodstream as they help to prevent further infection of non-infected cells.

4. Physiological barriers: Physiological barriers include body fluids, secretions like gastric juices, tears and certain chemical messengers that mediate immunity. For example, stomach juices have hydrochloric acid that will kill the microorganisms.


Acquired Immunity

Acquired immunity is the specific immunity developed by our body against a particular invader. In other words, it gets activated when the body is exposed to a threat. For instance, when our body gets exposed to TB organisms, it will respond by producing specific antibodies against the infection. These antibodies will store in the memory system of the immune system and whenever there is concurrent TB infection, then the memory gets activated to identify and attack the TB organisms. It is also known as adaptive immunity. It is further classified into active and passive immunity. Active Immunity is developed by our own body cells in the form of antibodies in response to a specific infection. Whereas Passive immunity is so-called because we need to administer some vaccines against a particular disease-causing agent. In the case of passive immunity, the body doesn`t actively involve in producing the antibodies.

B-lymphocytes and T-lymphocytes are the major mediators of acquired immunity because they help in producing antibodies that play a vital role in immunity. Antibodies are also known as immunoglobulins, which has 2 parts namely immune and globulin. Globulin is a form of protein that has 4 polypeptides with 2 heavy chains and the remaining 2 are of light chains joined to form a "Y" shaped molecule as shown below.


Each antibody differs due to the variation in their amino acid sequence especially at the tips of the "Y". This variable region has around 110-130 amino acids that provide a strong binding capacity with an antigen when trapped. The variable regions are the ends of the light and heavy chains. The remaining portions (apart from the tip) are called constant regions that determine the mechanism used to destroy the antigen. In human beings, natural antibodies are classified into 5 major classes namely, IgM, IgG, IgA, IgD, and IgE. All these antibodies are present in the blood, hence the response produced is known as the humoral immune response. On the other hand, there is a cell-mediated immune response or cell-mediated immunity which is driven by T –lymphocytes produced within the lymph glands. Apart from blood, lymph plays a vital role in immunity by producing lymphocytes.


Vaccination and Immunization

Until the middle of the 19th century, many were not aware of the fact that diseases can be prevented by injecting some antibodies-vaccines before the occurrence of illness. Lewis pasture, the famous biologist has introduced the concept of immunology. Immunization is a process by which a person is made stronger or resistant against a specific foreign body or an infection. It is done by the administration of a vaccine. Vaccines are substances that stimulate the production of antibodies and provide immunity against a particular illness or a group of illnesses. It is prepared by using inactivated, dead strains of bacteria or virus, in other words, the vaccine is derived from the disease-causing organisms itself by de-activating their virulence. Immunization or vaccination works on the principle of ‘memory’ of the immune system where antigenic proteins of a particular pathogen are made into an inactive or weak form. For this to happen, specific target pathogens will be introduced into the human body. Pathogens send a signal to the body regarding their entry, in response,  our immune system will boost the production of specific antibodies. When a vaccine is injected, they generate memory – B and T-cells that will recognize a particular pathogen. On their subsequent exposure, our system will quickly send a message to antibodies to act against invaders. There are various vaccines being used across the world based on the WHO guideline sheet.


Immune system

The immune system is not a single entity, but the combination of body systems work together to form an invisible component called the immune system. The human immune system has lymphoid organs like spleen, lymph nodes, thymus, bone marrow cells, blood cells and soluble molecules like antibodies. The immune system also plays an important role in allergic reactions, auto-immune diseases, and organ transplantation.


Lymphoid organs and their role in immunity

1. The primary organs of the lymphatic system are the lymph nodes. Lymph nodes are present in the form of small nodules in the limbs, especially more prominent in the groin and axilla. When an infective agent enters, they automatically trigger by producing lymph. Lymph nodes become red, enlarged and increase their circulation when they start acting against the foreign agents. 

2. The spleen plays a dual role of immunity as well as erythrocyte production. It is called the largest lymphatic organ in the body.  The spleen produces antibodies that will help to engulf bacterial and other microbes by a specific mechanism.

3. Bone marrow – it is produced in the hollow space of long bones like femur bone (thigh bone), humerus (upper arm) bone, etc. Special cells present inside the bones are known as stem cells which helps in blood cell production. Bone marrow indirectly participates in immune function by producing the blood cells, including the lymphocytes. In the bone marrow, the immature lymphocytes differentiate into antigen sensitive lymphocytes.

4. Thymus- The thymus is a present in the chest, below the neck. It usually disappears at puberty. The thymus is not a primary organ for immunity but it helps in producing cells called thymocytes that take part in immune function.




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