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Immunity – Definition, Types, Mechanism, Components, and Immunization

What is Immunity?

  • Immunity refers to the state of being insusceptible or resistant to harmful agents or processes, particularly pathogens or infectious diseases. The concept of immunity dates back to 430 B.C., but significant advancements were made in the 18th century, starting with Edward Jenner’s observations in 1798. Jenner noticed that milkmaids who had contracted the mild disease of cowpox were immune to smallpox, a highly deadly disease at the time.
  • Building upon Jenner’s findings, he inoculated cowpox pustules into an 8-year-old boy and intentionally infected him with smallpox. As predicted, the child did not develop smallpox. This pioneering work paved the way for the development of vaccines. Louis Pasteur further contributed to the field by creating vaccines for diseases such as chicken cholera, anthrax, and rabies.
  • In biology, immunity is the body’s ability to defend itself against disease-causing organisms. Every day, our bodies come into contact with numerous pathogens, but only a few actually result in diseases. This is because our bodies possess the ability to produce and release antibodies in response to these pathogens, thus protecting us from diseases. This defense mechanism is known as immunity.
  • Immunity can be acquired through two main methods: natural immunity and acquired immunity. Natural immunity is the innate resistance an individual possesses against certain pathogens due to factors like genetic makeup or physical barriers. Acquired immunity, on the other hand, is developed through prior exposure to a pathogen or through immunization.
  • When the body encounters a pathogen for the first time, it initiates an immune response. The immune system recognizes the foreign invader and produces specific antibodies to neutralize it. These antibodies are specialized proteins that bind to the pathogen and mark it for destruction by other immune cells. Additionally, immune cells called memory cells are created during this process, which remember the specific pathogen and enable a faster and more efficient response upon subsequent exposures. This is the basis of acquired immunity.
  • Immunization plays a crucial role in enhancing immunity. By introducing weakened or inactivated forms of pathogens, vaccines stimulate the immune system to produce an immune response without causing the actual disease. This process enables the body to develop memory cells and antibodies specific to the pathogen, providing long-term protection against future infections.
  • Overall, immunity is a vital aspect of our body’s defense system. It enables us to fight off harmful pathogens and prevents the development of various infectious diseases. Through the historical contributions of individuals like Jenner and Pasteur, as well as ongoing scientific advancements, our understanding and ability to harness immunity continue to improve, leading to better health outcomes for individuals and populations.

Definition of Immunity

Immunity is the body’s ability to resist or protect itself against harmful pathogens or diseases.

What is Immune System?

  • The immune system is the body’s most powerful defense mechanism, playing a crucial role in protecting us from harmful bacteria and maintaining overall health.
  • Immunology, a discipline of biology, focuses on the intricate workings of the immune system. Immunity refers to the ability of the body to combat antigens or pathogens, as well as the state of being healthy.
  • Comprised of cells, tissues, and organs, the immune system works collectively to safeguard the entire body. It employs various mechanisms to defend against invading germs.
  • The immune system possesses both innate and acquired abilities. Innate immunity is present from birth and provides immediate defense against pathogens. Acquired immunity, on the other hand, develops over time as the body is exposed to different pathogens or through immunization. Acquired immunity plays a role in conditions such as allergies, autoimmunity, and organ transplantation.
  • White blood cells, also known as leukocytes, are of utmost importance in the immune system. They have the capability to eliminate disease-causing germs and harmful chemicals.
  • In addition to leukocytes, the immune system also encompasses lymphoid organs, tissues, and specialized protein molecules called antibodies. These components work together to recognize and neutralize pathogens, preventing them from causing harm to the body.
  • The immune system’s intricate network of cells, tissues, and molecules allows it to effectively combat infections and maintain the body’s health. Ongoing research in immunology continues to enhance our understanding of this remarkable system and its role in preserving overall well-being.

Lymphoid Organs

  • Lymphoid organs refer to the immune system organs that defend the body against invading microorganisms that cause illnesses or the spread of cancer.
  • It consists of bone marrow, blood vessels, lymph nodes, lymphatic vessels, the thymus, the spleen, and other lymphoid tissue clusters.
  • Lymphoid organs are where lymphocytes originate, mature, and proliferate. They are classified as primary, secondary, or tertiary according to their degree of development and maturity.
  • These organs are composed of fluid connective tissues containing various leukocytes or white blood cells. White blood cells or leukocytes contain the highest proportion of lymphocytes.

Primary lymphoid organs

  • The primary lymphoid organs create lymphocytes and enable for their development. Additionally, it produces lymphocytes from immature progenitor cells.
  • Consequently, it is known as the core lymphoid organs. Among the principal lymphoid organs are the thymus and bone marrow.

Secondary lymphoid organs

  • The secondary lymphoid organs are referred to as peripheral lymphoid organs because they promote the places where lymphocytes interact with antigen to become effector cells. They launch an adaptive immune response.
  • These are the secondary lymphoid organs The spleen, tonsils, lymph nodes, and appendix, among others, are examples of secondary lymphoid organs.

Tertiary lymphoid organs

  • Typically, tertiary lymphoid organs contain a small number of lymphocytes. It plays a crucial function in the inflammatory process.

How does Immunity Work?

  • The understanding of how immunity works began to emerge through the pioneering work of scientists in the late 19th century. Emil von Behring and Shibasaburo Kitasato conducted experiments in 1890 that provided the first glimpse into the mechanism of immunity. They demonstrated that serum, a component of blood, contained substances that acted like antibodies. These antibody-like substances were capable of protecting against infections, thus laying the foundation for the concept of humoral immunity.
  • Emil von Behring’s significant contributions to immunology were recognized when he was awarded the Nobel Prize in Medicine in 1901 for his work on serum therapy, particularly for developing a treatment for diphtheria using antibodies.
  • Prior to von Behring’s discovery of serum components, Elie Metchnikoff had already made important observations about the role of cells in the immune system. In 1884, Metchnikoff demonstrated that certain white blood cells, which he called phagocytes, had the ability to engulf and digest bacteria and other foreign substances through a process known as phagocytosis.
  • Metchnikoff noticed that immunized animals had highly active phagocytic cells, leading him to conclude that these cells, rather than serum components, played a central role in mediating immunity. He identified the active phagocytic cells in the blood as monocytes and neutrophils, two types of white blood cells involved in immune responses.
  • Metchnikoff’s observations added a crucial understanding of cellular immunity, complementing Behring’s findings on humoral immunity. Together, these discoveries paved the way for further investigations and advancements in immunology.
  • In summary, the early work of scientists like Emil von Behring and Shibasaburo Kitasato shed light on the presence of antibody-like substances in serum, forming the basis of humoral immunity. Meanwhile, Elie Metchnikoff’s studies on phagocytic cells revealed the crucial role of cellular immunity in defending against infections. These groundbreaking discoveries laid the foundation for our current understanding of how immunity works, providing a framework for ongoing research in immunology.

Mechanism of Immunity

  • The mechanism of immunity became clearer through the groundbreaking experiments conducted by Emil von Behring and Shibasaburo Kitasato in 1890. Their work provided the initial understanding of how immunity functions. They demonstrated that serum, a component of blood, contained specific elements called antibodies. These antibodies played a crucial role in protecting against infections, thus establishing the concept of humoral immunity.
  • Emil von Behring’s remarkable contributions to the field of immunology were duly recognized when he was awarded the Nobel Prize in Medicine in 1901 for his pioneering work. His research and discoveries significantly advanced our understanding of the immune system and its mechanisms.
  • Even before von Behring’s breakthrough with serum components, Elie Metchnikoff had made important observations regarding the role of cells in immune responses. In 1884, Metchnikoff demonstrated that certain types of white blood cells, which he named phagocytes, had the ability to engulf and destroy microorganisms and other foreign materials through a process known as phagocytosis.
  • Metchnikoff noticed that immunized animals exhibited highly active phagocytic cells. Based on this observation, he hypothesized that cells played a more prominent role in mediating immunity compared to serum components. He identified these active phagocytic cells in the blood as likely being monocytes and neutrophils, two types of white blood cells involved in immune responses.
  • Metchnikoff’s findings contributed significantly to the understanding of cellular immunity, complementing von Behring’s discoveries regarding humoral immunity. Together, their work provided a comprehensive framework for comprehending the mechanisms underlying immunity.
  • In summary, the pioneering experiments conducted by Emil von Behring and Shibasaburo Kitasato shed light on the presence of antibodies in serum, elucidating the foundations of humoral immunity. Meanwhile, Elie Metchnikoff’s research demonstrated the crucial role of phagocytic cells in immune responses, highlighting the significance of cellular immunity. These breakthroughs laid the groundwork for our current understanding of the mechanisms that govern immunity and continue to guide further advancements in the field of immunology.

Types of Immunity 

The primary purpose of the immune system is to prevent or restrict infections caused by harmful microorganisms such as bacteria, viruses, parasites, and fungi. A host’s immune responses commence with the identification of bacteria and foreign chemicals. The body’s defence systems are (a) innate (natural) immunity and (b) acquired (adaptive) immunity.

1. Innate Immunity 

  • This sort of immunity is present at birth in an organism.
  • This is immediately engaged when a pathogen is detected. Certain barriers and defence systems that keep foreign particles out of the body are a part of innate immunity.
  • Innate immunity is the body’s defence mechanism.
  • This immunity benefits us by supplying the natural resistance components, such as salivary enzymes, natural killer cells, undamaged skin, and neutrophils, etc., that form an early reaction against illnesses at birth before exposure to pathogens and antigens.
  • It is a form of long-term immunity in which our body develops its own antibodies. Few natural barriers prevent viruses from entering the body.

What is Innate immune responses?

Innate immunity is regarded as the host’s initial line of protection against foreign agents. There are five categories of innate immune response components:

  • The removal of foreign agents from body tissues by macrophages, which are non-specific white blood cells.
  • By generating anti-inflammatory mediators known as cytokines and chemokines, immune cells are attracted to the site of infection.
  • The complement cascade is a chain of immunological processes that functions to eliminate germs, infected host cells, and debris.
  • Additionally, innate immunity serves as a physical barrier against the entrance of foreign agents (e.g. skin is a component of the innate immune system).
  • By delivering foreign, processed antigens to adaptive immunity, the innate immune response activates the adaptive immune system.

Cells Involved In Innate Immunity

  • Phagocytes: These cells circulate throughout the body and search for foreign substances. They envelop and destroy the infection to protect the body against it.
  • Macrophages: These cells are capable of traversing the walls of the circulatory system. They secrete cytokines to recruit additional cells at the location of an infection.
  • Mast cells: Mast cells are essential for wound healing and defending against infections.
  • Neutrophils: Neutrophils have naturally poisonous granules that kill any infection with which they come into contact.
  • Eosinophils: Eosinophils contain highly poisonous proteins that eliminate any germs or parasites that come into touch with them.
  • Basophils: These cells target multicellular parasites. Similar to mast cells, these cells produce histamine.
  • Natural Killer Cells: These destroy infected host cells to prevent the spread of diseases.
  • Dendritic Cells: These are found in the tissues that serve as first infection entry locations. By antigen presentation, these cells alert the rest of the immune system to the presence of an infection.

Types of Barriers

The four types of barriers are:

a. Physical barrier

  • This includes the skin, body hair, cilia, eyelashes, respiratory system, and digestive system.
  • These constitute the initial line of defence.
  • The skin provides more than just a fair or dark complexion. Our skin serves as a physical barrier against virus infiltration.
  • The mucus coating in the nose and ear acts as a protective barrier that captures pathogens before they can enter the body.

b. Physiological barriers

  • We know that hydrochloric acid is used by the stomach to break down food molecules.
  • Due to such a highly acidic environment, the majority of pathogens that enter the body with meals are eliminated prior to further processing.
  • Saliva in our mouth and tears in our eyes both have the antibacterial property that prevents the growth of germs despite constant exposure.

c. Cellular barriers

  • Despite physical and physiological barriers, some infections are able to infiltrate the human body.
  • Leukocytes (WBC), neutrophils, lymphocytes, basophils, eosinophils, and monocytes comprise this barrier. Every one of these cells is present in the blood and tissues.

d. Cytokine barriers

  • Our body’s cells are more intelligent than we give them credit for. For instance, whenever a cell in our body is invaded by a virus, it automatically secretes interferons, which form a coating around the diseased cell and inhibit further infection of the surrounding cells.

2. Acquired Immunity

  • Adaptive immunity, also known as acquired immunity, is the immunity that our body gains or acquires over time. This, unlike innate immunity, is not present at birth.
  • Acquired immunity is the capacity of the immune system to adapt to illness and create pathogen-specific immunity. This is also called adaptive immunity.
  • An individual gets immunity after birth; hence, it is known as acquired immunity.
  • It is particular and mediated by lymphocytes or antibodies that render the antigen harmless.
  • The primary goal of acquired immunity is to alleviate the symptoms of an infectious disease and prevent subsequent attacks.
  • It comprises primarily of a sophisticated lymphatic defence system that recognises and does not react to the body’s own cells.
  • The immune system of our body recognises germs it has already encountered. It is generally caused by direct contact with the infection or its antigen.
  • Our body produces antibodies to engulf and destroy the pathogen’s antigen.
  • When something is encountered for the first time, it is referred to as a primary response.
  • Once the body becomes accustomed to certain pathogens, antibodies are prepared to battle them again; this is known as naturally acquired immunity.

Features of Acquired Immunity

  • Specificity: Our body is able to distinguish between different sorts of infections, whether they are hazardous or not, and design methods to eliminate them.
  • Diversity: Our immune system can detect a wide range of infections, from protozoa to viruses.
  • Differentiate between self and non-self: Our body is able to distinguish between its own cells and foreign ones. It begins rejecting any foreign cells in the body instantly.
  • Memory: Once the body detects a pathogen, it activates the immune system to eliminate it. It also recalls which antibodies were produced in response to a particular infection so that, the next time the pathogen enters the body, a similar process is followed to destroy it.

What is Adaptive immune Responses?

  • This is the body’s most specialised immune response.
  • With the support of two primary immune cells, B-cells and T-cells, it seeks to eliminate previously encountered foreign invaders.
  • B cells are responsible for the release of foreign antigen-specific antibodies.
  • T cells are responsible for both activating B cells (T helper cells) and destroying pathogens/host cells that have been infected by pathogens (T-killer cells).
  • This branch of the immune system is adaptive due to the multiple processes it employs to mature immune cells, so rendering them highly specific to antigens encountered on infections and presented by the innate immune system.
Acquired Immunity
Acquired Immunity

Cells Involved in Acquired Immunity

There are two types of cells involved in acquired immunity: B-cells and T-cells.


  • They are generated in the bone marrow.
  • These cells become active upon contact with external substances. These alien particles serve as identifiers for foreignness.
  • B-cells rapidly develop into plasma cells that produce antibodies specific to the so-called antigen or foreign particle.
  • These antibodies connect to the antigen/foreign agent’s surface.
  • These antibodies can identify and eliminate any antigen in the body.
  • Immunity reliant on B-cells is known as humoral immunity.


  • They form in the thymus after developing in the bone marrow.
  • Helper cells, cytotoxic cells, and regulatory cells are the products of T-cell differentiation. The release of these cells into the bloodstream.
  • When activated by an antigen, these cells release cytokines that function as messengers.
  • These cytokines initiate the development of B-cells into plasma cells, which thereafter release antigen-specific antibodies.
  • The cancer cells are killed by cytotoxic T-cells.
  • Regulatory T-cells control immunological responses.

Types of Acquired Immune Response

Humoral Immune Response

  • Antibodies generated by B-lymphocytes are found in blood cells and are carried throughout the body. The humoral immune response is composed of an antibody produced by lymphocytes; hence, its name.
  • It depends on the action of circulating antibodies in the body. Humorous immunity is activated when an antibody on a B-cell connects with an antigen. The B cell internalises the antigen and presents it to the T cell helper. This stimulates B-cells.
  • Activated B cells proliferate and generate plasma cells.
  • These plasma cells secrete antibodies into the circulatory system. The memory B cells maintain information about the infection in order to prevent future diseases caused by that pathogen.

Cell-mediated Immune Response

  • T helper cells start the process of cell-mediated immunity.
  • By producing toxins and consequently causing apoptosis or programmed cell death, cytotoxic T cells eradicate infected cells from the body.
  • T helper cells contribute to the activation of other immune cells. In the case of transplant patients, cell-mediated immunity becomes obvious.
  • When one of our sense organs ceases to function, another organ can be transplanted to replace it. However, this is not the case with the immunological response. It suggests that T-lymphocytes are capable of distinguishing between native and foreign tissues and organs.
  • Because our body may reject the transplanted organ, even if we find a donor with the same blood group, we cannot transplant and implant organs into our bodies.
  • T-cells instantly identify the tissue or organ as alien and prevent it from becoming a part of the body.
  • This is the reason why transplant recipients must take immunosuppressants for the rest of their lives.
  • T-lymphocytes are responsible for controlling this response.

Types of Acquired Immunity

Active Immunity

  • Active immunity is the immediate response of the body to a foreign antigen.
  • In the acquired or adaptive immune system, the body recalls the infections it has previously faced. This is directly attributable to an active immune system.
  • Active immunity occurs when the disease or its antigen is encountered.
  • Antigens stand for antibody generator.
  • With the aid of antigens secreted by the disease, our body is able to combat the pathogen.
  • Consequently, based on the antigen of the virus, our body begins generating antibodies to combat it.
  • When this occurs initially, it is referred to as a primary response.
  • Once the body has encountered a virus for the first time, it retains a few of the antibodies that fought it in case it assaults again. The term for this is natural active immunity.

Passive Immunity

  • Passive immunity is the immunological response caused by antibodies acquired from the outside of the body. The body’s initial response to a disease it encounters for the first time is quite poor, hence the initial encounter is always somewhat taxing on the body.
  • What if everyone could be immunised without ever having to become sick? Over the past two decades, biotechnology has advanced significantly, and we are now able to manufacture antibodies against diseases. Even if the body has not yet mounted a major immune response, these premade antibodies offer protection.
  • Active immunity may provide lifelong protection against a disease, but passive immunity is more temporary.
  • Passive immunity develops instantly, allowing our body to quickly begin attacking the invader.
  • Two types of passive immunity exist:
Innate and Adaptive Immunity
Innate and Adaptive Immunity


What is immunity?

Immunity refers to the body’s ability to defend itself against harmful pathogens, such as bacteria, viruses, and parasites, and protect against diseases.

How does the immune system work?

The immune system is a complex network of cells, tissues, and organs that work together to identify and eliminate foreign substances. It includes white blood cells, antibodies, and various immune molecules.

What are the types of immunity?

There are two main types of immunity: innate immunity and adaptive immunity. Innate immunity is present from birth and provides a general defense against pathogens. Adaptive immunity develops over time, as the body learns to recognize and respond to specific pathogens.

What are antibodies?

Antibodies, also known as immunoglobulins, are proteins produced by the immune system in response to the presence of antigens (foreign substances). They bind to specific antigens and help in neutralizing or eliminating them.

How does vaccination work?

Vaccination stimulates the immune system by introducing weakened or inactive forms of pathogens or their components. This exposure triggers an immune response, leading to the production of antibodies and the development of memory cells for long-term protection.

Can lifestyle choices affect immunity?

Yes, certain lifestyle choices can impact immunity. Maintaining a balanced diet, regular exercise, adequate sleep, stress management, and avoiding smoking and excessive alcohol consumption can contribute to a healthy immune system.

What is immunodeficiency?

Immunodeficiency refers to a weakened or compromised immune system, which can be acquired or inherited. It increases susceptibility to infections and makes it harder for the body to fight off diseases.

Can stress affect immunity?

Yes, chronic stress can weaken the immune system and make individuals more vulnerable to infections. Managing stress through relaxation techniques, exercise, and healthy coping mechanisms can help support immune function.

How long does immunity last after an infection or vaccination?

The duration of immunity can vary depending on the disease or vaccine. Some infections confer lifelong immunity, while others may provide temporary or waning immunity. Vaccinations may require booster doses to maintain long-term protection.

Can you boost your immune system?

While you cannot instantly boost your immune system, adopting a healthy lifestyle can support its optimal functioning. This includes a nutritious diet, regular exercise, adequate sleep, stress management, and avoiding harmful habits.


  • Kuby Immunology 7th Edition
  • Microbiology by Prescott
  • Microbiology and Immunology 2nd Edition by Shubash Chandra Parija
  • Rodgers, J. R. (2009). Immunity. Encyclopedia of Microbiology, 481–499. doi:10.1016/b978-012373944-5.00191-7
  • Immunity. (2011). Pathology Illustrated, 87–111. doi:10.1016/b978-0-7020-3376-6.50009-1

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