Concept of immunity


Immunity is the ability of the body to defend against invaders; foreign bodies and microorganisms. Each one of us have a specific level of immunity that helps in blocking the invasion of disease-causing agents from outside. One`s immunity level depends upon the age, general health status, nutritional status, sociocultural practices and the genetic influences among many others. Our body is capable of regenerating the specific antibodies to fight against the antigens seldom, few individuals might devoid of this ability. Based on the ability to naturally defend against the disease-causing agents, people might either experience frequent infections or may not for a period of time. Nevertheless, if the invading organisms are stronger, and the other risk factors are in favor of the infections agents, disease is unavoidable. Moreover, individuals respond differently to the same organisms under similar circumstances. To put it another way, each one of use differ when it comes to the level of immunity . Hence, one may feel safe in the dusty weather whereas the other individual might fall sick. Let us understand the basic concepts and types of immunity. 



"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 defense mechanism present in an individual by birth itself. In a healthy individual, the body has the natural capacity to fight against the invading microorganisms, however, in the immunocompromised patients, the body fails to do so. On the other hand, adaptive immunity or acquired immunity is an artificially induced immunity with the help of vaccines. Vaccines are the substances that boosts the body`s natural defense mechanism by producing specific antibodies. Some vaccines are BCG, DPT, DT, TT, MMR, oral polio vaccine, Hepatitis vaccine and cholera vaccine.


Types of immunity



Natural Immunity or innate immunity

1. Physical barriers: Some physical barriers help in safeguarding our body are the skin, mucous membrane, and hair. They protect us by making a barrier against the invaders. They have the ability to identify the antigens  such as allergens, microorganisms and toxins. Among all the barriers, the skin is one of the most helpful organs to fight illnesses. The epidermis, the outer layer of the skin is very thick and strong to resist the entry of most of the microorganisms. Similarly, the mucus membrane produces thick secretions that can trap the dust and microorganisms.

2. Cellular barriers: The major cellular barriers that help in the immune mechanisms are WBCs, lymphocytes, polymorphonuclear leukocytes or neutrophils and monocytes. The cellular barriers work by imitating phagocytosis - a process of destroying foreign antigens. The cells  of our body systems can defend against almost all the microorganisms with the help of phagocytosis.

3. Cytokine barriers: Here the immunity is mediated by interferons. They are named so for their ability to "interfere" with viral replication by protecting cells from virus infections. Interferons are proteins secreted by cytokines. They can quickly detect when a foreign agent enters into the body by establishing communication between the body cells and the foreign agents.

4. Physiological barriers: Physiological barriers include body fluids, secretions like gastric juices, tears and certain chemical messengers that mediate immunity. For example, HCL present in the stomach juices can kill the microorganisms.

Acquired Immunity

Acquired immunity is also known as adaptive immunity. It is a special ability developed artificially by our body hence it is not a natural immunity. In other words, it is activated when the body is exposed to a threat. For instance, when our body comes in contact with the TB organisms, it will respond by sending some antibodies to the site of infection. These antibodies get used to identify the organisms, as a result, they quickly form a network of protective agents as soon as the TB organisms come in contact. Acquired immunity is further classified into active and passive immunity. Active Immunity is developed by the body cells while they come in contact with  foreign agents hence they need not be injected in the form of vaccines. On the contrary, Passive immunity demands some intervention in the form of prophylaxis or vaccines. Passive immunity is again classified into natural passive immunity and artificial passive immunity. The natural passive immunity is mainly mediated by B-lymphocytes and T-lymphocytes.  However, many other cells in our body participate in the process, for example, a newborn baby acquires natural passive immunity is transmitted by the mother through the placenta. Artificially-acquired passive immunity on the other side is, an immediate, but short-term protection provided by the injecting the artificially prepared antibodies.  Antibodies are also known as immunoglobulins.


Structure of Immunoglobulins

Immunoglobulins are the proteins , containing 4 polypeptides with 2 heavy chains and 2 light chains joined to form a "Y" shaped molecule as shown below. Antibody slightly differ from individual to individual due to the variation in their amino acid sequence near the tip of the Y- junction.  There are 2 regions in an immunoglobulin, variable region and the constant region. Variable region comprises 110-130 amino acids, and they provide the antibody its specificity to binding capacity ainst the antigen.  On the other hand, the constant region determines the mechanism employed to destroy the antigen. Immunoglobulins are divided into 5 major classes, IgM, IgG, Iga, IgD, and IgE, based on their constant region structure and immune function. 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. Aside from blood, lymph also plays a vital role in our body`s immunity by producing lymphocytes.




Until the middle of the 19th century, many were unaware of the fact that diseases can be prevented by injecting some vaccines. Lewis pasture, the famous biologist has introduced the concept of immunology. Immunization is a process by which a person is made resistant against a specific foreign body with the help of vaccines. Immunization is mediated by the artificial immune-boosters called vaccines. Vaccines are substances that stimulate the production of antibodies  against a particular illness or a group of illnesses. Antibodies are prepared by using inactivated, dead strains of bacteria or virus. A vaccine is derived from the disease-causing organisms itself by deactivating their ability to harm us. When they enter our body, pathogens send a signal to the body about their entry, in response,  our immune system will boost the production of specific antibodies. Vaccines help to produce specific target antibodies against a particular pathogen. 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 made up of special organs, cells and chemicals to fight the specific antibodies released by microbes. The fundamental parts of the immune system are white blood cells, the lymphatic system, immunoglobulins released by the body. Lymphatic system mainly comprises the complement system supported by the spleen, thymus, and the bone marrow. Let us brief the role of various organs in our body`s immunity.


Major Lymphoid organs

1. Lymph nodes

Lymph nodes are ubiquitous, and they take the form of mall nodules at the particular junctions such as underarms, groin, and part of the lower limbs. When an infective agent enters in to our body, they automatically trigger a response by sending immunoglobulins. Our lymph nodes become red, enlarged and very active while fighting against the foreign agents. 

2. The spleen 

The spleen is a blood-filtering organ aid in removing the microbes. They also assist in cleaning old or damaged red blood cells produced at the time of defence. It also offers help by boosting the energy for antibodies and lymphocytes.

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, they help in the production of blood cells. 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 specialized primary lymphoid organ of the immune system. Within the thymus, T-cells mature. T-cells are critical to the adaptive immune system, where the body adapts specifically to foreign invaders.


Further reading






Blood and circulatory system          


Human skin'



  1. Define immunity.

  2. What are antigens and antibodies pertaining to immunity?

  3. Mention the different organs responsible for immunity in our body.

  4. Explain the role of vaccines in acquiring immunity.

  5. What is the role of WBCs in immunity?

  6. Explain the structure of immunoglobulins.


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