IMMUNITY
Introduction
- The human body is constantly exposed to infectious agents, but not all exposures lead to disease.
- Immunity is the body’s ability to defend itself against disease-causing organisms.
Types of Immunity
- Immunity can be categorized into two types:
- Innate Immunity
- Non-specific defense mechanisms present at birth.
- Includes physical, physiological, cellular, and cytokine barriers.
- Acquired Immunity
- Pathogen-specific and characterized by memory.
- Involves primary and secondary immune responses.
- Mediated by B-lymphocytes and T-lymphocytes.
- Innate Immunity
Innate Immunity
- Innate immunity provides non-specific defense against foreign agents.
- Four types of barriers contribute to innate immunity:
- Physical barriers: Skin, mucus coating, and epithelium lining prevent microbial entry.
- Physiological barriers: Stomach acid, saliva, and tears inhibit microbial growth.
- Cellular barriers: Leukocytes, such as polymorpho-nuclear leukocytes, monocytes, natural killer cells, and macrophages, can phagocytose and destroy microbes.
- Cytokine barriers: Virus-infected cells secrete interferons, protecting non-infected cells from viral infection.
Acquired Immunity
- Acquired immunity is pathogen-specific and characterized by memory.
- The body’s response to a pathogen can be divided into a primary response (low intensity) and a secondary response (highly intensified).
- The primary and secondary immune responses involve B-lymphocytes and T-lymphocytes in the blood.
Antibody-Mediated Immune Response (Humoral Immune Response)
- B-lymphocytes produce antibodies (proteins) in response to pathogens.
- Antibodies, such as IgA, IgM, IgE, IgG, are produced in the blood.
- Antibodies help fight pathogens in the bloodstream.
- The acquired immune response mediated by antibodies is known as the humoral immune response.
Cell-Mediated Immune Response (Cell-Mediated Immunity – CMI)
- T-lymphocytes mediate cell-mediated immunity.
- In cases like organ transplantation, the cell-mediated immune response is responsible for graft rejection.
- The body can differentiate between ‘self’ and ‘nonself,’ leading to the rejection of grafts.
Active and Passive Immunity
Active Immunity
- Active immunity is a type of immunity in which the host’s immune system produces antibodies in response to exposure to antigens, which can be living or dead microbes or other proteins.
- It is characterized by the host’s body actively mounting an immune response against the antigens.
- Active immunity is a slow process and takes time to develop its full protective response.
- It can be induced through two primary methods:
- Immunization: Deliberate injection of a weakened or killed form of a microbe or an antigen to stimulate the immune system. This process helps the body produce antibodies and memory cells to defend against future infections.
- Natural Infection: Exposure to infectious organisms naturally, allowing the immune system to develop immunity over time.
Passive Immunity
- Passive immunity is a type of immunity in which ready-made antibodies are directly introduced into the host’s body to protect against foreign agents.
- In passive immunity, the host does not produce its own antibodies but receives them from an external source.
- Passive immunity provides immediate protection against specific antigens.
- Examples of passive immunity include:
- Mother’s Milk: Colostrum, a yellowish fluid secreted by a mother during the initial days of lactation, contains abundant antibodies (IgA) that help protect the newborn infant.
- Placental Transfer: During pregnancy, antibodies from the mother can pass through the placenta to the developing fetus, providing temporary immunity to the newborn.
Significance of Passive Immunity
- Passive immunity is essential in situations where immediate protection is needed or when the host’s immune system is unable to produce an adequate response.
- It serves as a temporary defense mechanism until the host’s own immune system can develop active immunity.
Vaccination and Immunization
The Principle of Immunization
- Immunization or vaccination is based on the immune system’s ability to remember past encounters with pathogens.
- Vaccination introduces specific antigenic proteins of a pathogen or inactivated/weakened pathogens (vaccine) into the body.
Active Immunity through Vaccination
- Antibodies produced in response to these antigens in the vaccine neutralize the pathogenic agents during an actual infection.
- Vaccines also establish memory in the immune system, including B and T-cells that can recognize the pathogen quickly upon subsequent exposure.
- Memory cells result in an accelerated immune response, leading to a rapid and robust production of antibodies.
Passive Immunization
- In some cases, such as severe infections like tetanus or snakebites, immediate immune response is necessary.
- Preformed antibodies, also known as antitoxins, are directly injected into the patient’s body to counteract the deadly microbes or toxins.
- This type of immunization, where preformed antibodies are administered, is called passive immunization.
Application of Recombinant DNA Technology
- Recombinant DNA technology has revolutionized the production of antigenic polypeptides of pathogens.
- Antigenic polypeptides can be produced in bacteria or yeast using this technology.
- Recombinant DNA technology allows large-scale production of antigens, ensuring greater availability for immunization purposes.
- An example of a vaccine produced using this approach is the hepatitis B vaccine, which is produced from yeast.
Allergies
Sensitivity to Environmental Allergens
- Some individuals experience unexpected sneezing, wheezing, and other symptoms when they visit new places but find relief upon leaving.
- These reactions may be due to sensitivity to environmental particles, such as pollen, mites, and other allergens that vary by location.
What is an Allergy?
- Allergy is an exaggerated response of the immune system to specific antigens present in the environment.
- Substances that trigger such immune responses are called allergens.
- Allergens often lead to the production of IgE antibodies.
Common Allergens
- Common examples of allergens include dust mites, pollen, animal dander, and more.
- Exposure to these allergens can result in allergic reactions with symptoms like sneezing, watery eyes, a runny nose, and difficulty in breathing.
Mechanism of Allergic Reactions
- Allergic reactions are caused by the release of chemicals, such as histamine and serotonin, from mast cells.
- These chemicals trigger the typical allergy symptoms.
Diagnosis and Treatment
- To identify the specific cause of an allergy, patients may be exposed to or injected with small amounts of potential allergens, and their reactions are studied.
- Medications like anti-histamines, adrenaline, and steroids are commonly used to alleviate allergy symptoms.
Increasing Allergies in Modern Life
- The modern lifestyle and the urban environment have led to a decrease in immunity and an increase in sensitivity to allergens.
- More children in metro cities in India are suffering from allergies and asthma due to environmental sensitivities.
- This could be attributed to a protected early environment that may result in decreased immune system exposure to common allergens.
Auto Immunity
Background
- Memory-based acquired immunity in higher vertebrates has evolved to differentiate between foreign organisms (e.g., pathogens) and self-cells.
- While the exact basis of this ability is not fully understood, two important corollaries of this immune response are recognized.
Corollary 1: Distinguishing Foreign Molecules
- Higher vertebrates possess the ability to distinguish not only foreign organisms but also foreign molecules.
- This capability forms a significant area of study in experimental immunology.
Corollary 2: Autoimmune Diseases
- In some cases, the immune system may erroneously target and attack the body’s own cells.
- This self-attack results in damage to the body and is known as an autoimmune disease.
Autoimmune Diseases
- Autoimmune diseases occur when the immune system mistakenly identifies self-cells or tissues as foreign and mounts an immune response against them.
- An example of an autoimmune disease is rheumatoid arthritis, a condition that affects a significant number of people in society.
Immune System in the Body
Components of the Immune System
- The human immune system is composed of various elements, including lymphoid organs, tissues, immune cells, and soluble molecules like antibodies.
- The immune system possesses the unique ability to recognize foreign antigens, respond to them, and remember them.
- It plays a crucial role in various physiological processes, including allergic reactions, autoimmune diseases, and organ transplantation.
Lymphoid Organs
- Lymphoid organs are the sites where the origin, maturation, and proliferation of lymphocytes take place.
- Primary lymphoid organs include the bone marrow and thymus, where immature lymphocytes differentiate into antigen-sensitive lymphocytes.
- After maturation, lymphocytes migrate to secondary lymphoid organs like the spleen, lymph nodes, tonsils, Peyer’s patches in the small intestine, and the appendix.
- Secondary lymphoid organs provide locations for lymphocytes to interact with antigens, leading to their proliferation and transformation into effector cells.
Key Lymphoid Organs
- Bone Marrow: The primary organ responsible for the production of all blood cells, including lymphocytes.
- Thymus: A lobed organ located near the heart and beneath the breastbone. It provides an environment for the development and maturation of T-lymphocytes.
- Spleen: A large bean-shaped organ containing lymphocytes and phagocytes. It filters the blood by trapping blood-borne microorganisms and also serves as a reservoir for erythrocytes.
- Lymph Nodes: Small, solid structures situated along the lymphatic system, which trap microorganisms and other antigens from lymph and tissue fluids. Antigens trapped in lymph nodes activate lymphocytes, leading to an immune response.
Mucosa-Associated Lymphoid Tissue (MALT)
- Lymphoid tissue is also found within the lining of major tracts, such as the respiratory, digestive, and urogenital tracts.
- This tissue is known as mucosa-associated lymphoid tissue (MALT) and constitutes approximately 50% of the lymphoid tissue in the human body.
