Concept of immunity

Immunity is the ability of the body to defend against disease causing invaders such as  foreign bodies and microorganisms. Each one of us has a specific level of immunity that helps in blocking the invasion of these disease-causing agents. The difference in the levels of immunity depends upon the age, general health status, nutritional status,  sociocultural practices and the genetic influences among many others.

Disease causation

In most cases, our body is capable of generating the specific antibodies to fight against the antigens however, in certain circumstances, a few individuals fail to do this. Diseases occur depending upon one`s  ability to naturally defend against the disease-causing agents. On the other side, if the invading organisms are too strong than the host, it favours agents. Some people respond poorly despite their ability to produce natural immunity. This may be connected with various other biological, familial, geographical and psychological factors. Many bacteria and viruses coexist with the host, causing disease when  the host is compromised by impaired immune defense mechanisms. By contrast, the most virulent pathogens can produce disease in almost every infected host.

Types of immunity

There are two types of immunity: innate immunity, and adaptive immunity. Innate immunity is a natural defense mechanism existing by birth itself. Our body in general, has the natural capacity to fight against the invading microorganisms, however, in the immunocompromised patients, it fails to do so. On the other hand, adaptive immunity or acquired immunity is an artificially induced immunity with the help of vaccines. Vaccines boosts the body`s natural defence mechanism by producing specific antibodies. Some common vaccines include BCG, DPT, DT, TT, MMR, OPV(oral polio vaccine), Hepatitis vaccine and cholera vaccine.

 

Natural Immunity or innate immunity

Innate immunity or natural immunity refers to nonspecific defence mechanisms that start immediately or within hours of the entry of antigens. Mechanisms that include natural immunity are  physical barriers, cellular barriers, cytokine barriers, and physiological barriers:

1. Physical barriers

 Some physical barriers help in safeguarding our body are the skin, mucous membrane, and hair. They create a barrier by identifying the invaders early. Our skin is one of the most helpful organs to fight illnesses. The epidermis- outer layer of the skin is very thick and does not easily allow the innecessary entry of microbes. Similarly, the mucus membrane produces thick sticky secretions that can trap the dust and microorganisms.

2. Cellular barriers

 The major cellular barriers that help in the immune mechanisms are WBCs, lymphocytes, neutrophils and monocytes. The cellular barriers operate by phagocytosis. Phagocytes ingest or engulf other cells or particles that form antigens. The cells  of our body systems can defend against almost all the microorganisms with the help of phagocytosis.

3. Cytokine barriers

The immunity produced by cytokine barriers is mainly because of interferons. Interferons are named so for their ability to "interfere" with viral replication hence it is one of the best way to block viral infections. Interferons are the proteins secreted by cytokines and 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

Anatomical and physiological barriers provide the most essential first-line of defense against pathogens. Some of them include vigorous mucociliary clearance mechanisms, low stomach pH and bacteriolytic lysozyme in tears, salivary action  and physiological barriers created by other body fluids.

Adaptive or Acquired Immunity

Acquired immunity is also known as adaptive immunity. It is an artificially developed ability by our body when exposed to a threat. The process begins when the antibodies present in our body acknowledges a real danger by the outside agents. For instance, when our cells gain contact to TB organisms, some antibodies found in our blood reaches the site of infection to initiate antigen-antibody reactions. These antibodies develops a mechanism to identify the infection causing agents, following which , they form a network of protective shield against the TB organisms. Acquired immunity is classified into active and passive immunity. Active immunity occurs when an individual is able to produce antibodies against a disease through his or her own immune system. Passive immunity is developed when a person is artificially injected by antibodies. Passive immunity is again classified into natural passive immunity and artificial passive immunity. The natural passive immunity is mainly mediated by cells of blood and lymph. Some of them include B-lymphocytes and T-lymphocytes. Artificially-acquired passive immunity on the other side is, an immediate but short-term protection provided by injecting the artificially prepared immunoglobulins.

Immunoglobulins

Immunoglobulins, also known as antibodies, are glycoprotein molecules produced WBCs. They play a crucial part in the immune response by specifically recognizing and binding to particular antigen produced by bacteria or viruses. Immunoglobulins are divided into 5 major classes, IgM, IgG, Iga, IgD, and IgE, based on their constant region structure and immune function. Immunoglobulins contain 4 polypeptide chains: 2 heavy chains and 2 light chains and are joined to form a "Y" shaped molecule as shown in the image below. Antibodies of an individual are slightly different from other individuals due to the variation in their amino acid sequence near the tip of the Y- junction. 

Structure of Immunoglobulins

An immunoglobulin has 2 regions : variable region and the constant region. The variable region comprises 110-130 amino acids, they provide the attribute of specificity to the antibody where specificity refers to the ability of the immunoglobulins to bind to the specific antigens. On the other hand, a constant region provides the actual mechanism to destroy specific antigens. 2 types of imune responses produced by the immunoglobulin sinclude humoral and cell-mediated immunity. Since immunoglobulins are found in the blood, the response produced is known as the humoral immune response. Cell-mediated immune response or cell-mediated immunity is stimulated by T –lymphocytes produced by our lymph glands. 

 

Vaccination(Immunization)

Until the middle of the 19th century, there had been a little knowledge about vaccines. From the time Lewis pasture, the famous biologist has introduced the concept of immunology, many new vaccines have stepped into the global market. Immunization is a process of inducing a person`s body to make him resistant against a specific foreign. It is done with the help of vaccines. Vaccines are the artificial immune-boosters as they stimulate the production of antibodies  against a specific illness. Antibodies are prepared by inactivating the dead strains of bacteria or virus so that they lose their ability to harm us while retaining their capacity to kill the antigens. When pathogens enter our body, the antibodies found in the vaccines sends the signal to the body to initiate quick response . Such response boost the production of specific antibodies depending upon th enature of antigens. When a vaccine is injected, they generate B-cells and T-cells that can recognize a particular pathogen and these cells retain this memory to help our body when the same antigens attack next time. 

Immune system

Our  immune system is made up of specialized cells that releases some chemicals to fight against the specific antibodies. Some of the fundamental components of the immune system are white blood cells, lymphatic cells, and immunoglobulins released by our body. The lymphatic system play a key role in natural immunity and it is constituted by the spleen, thymus, and the bone marrow.

Major Lymphoid organs

1. Lymph nodes

Lymph nodes are ubiquitous and are found as small nodules in different regions but are mainly distributed in the groin, and part of the lower limbs. A lymph node produces T-lymphocytes when an infective agent gains access into our body. Thymus is the largest lymphatic organ. Some other nodes that help in the immune function are tonsils, adenoids,  cervical nodes, Supraclavicular nodes, Axillary and  pectoral nodes.

2. The spleen 

The spleen is a blood-filtering organ aid in removing the microbes. It can also assist in cleaning old or damaged red blood cells through natural hemolysis. The organ synthesizes and removes antibody-coated bacteria and antibody-coated blood cells by way of blood and lymph node circulation. The spleen is a center of activity of the mononuclear phagocyte system and is equivalent to a large lymph node which means , its absence can predispose our body to certain infections

3. Bone marrow 

Bone marrow is produced in the hollow space of long bones like femur bone (thigh bone), humerus (upper arm) bone, etc. Stem cells found in the bone marrow help in the production of RBCs, thereby it can indirectly help in the immune function. 

4. Thymus

 The thymus is a specialized primary lymphoid organ of the immune system.it synthesizes T-cells which are critical to the adaptive immune system, where the body adapts specifically to foreign invaders.

 

Further reading

  1. Vaccines

  2. WBCs

  3. Blood and circulatory system 

  4. Human skin

Questions

  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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