6.1 Introduction to Immunity
5 min read•june 18, 2024
The immune system is our body's defense against invaders. It's a complex network of organs, cells, and proteins that work together to protect us from harmful pathogens like and . Understanding its components and functions is crucial for grasping how our bodies fight disease.
Immune responses come in two main flavors: antibody-mediated and cell-mediated. , produced by , neutralize threats in our blood and tissues. , on the other hand, directly attack infected or abnormal cells. Both types of are essential for our overall health and well-being.
Overview of the Immune System
Functions of immune system
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- Protects the body against invading pathogens such as bacteria, viruses, fungi, and parasites
- Distinguishes between self and non-self (foreign) substances to maintain immune tolerance
- Maintains homeostasis by removing damaged or abnormal cells through apoptosis or
- Components of the immune system
- Lymphoid organs
- Primary lymphoid organs: produces blood cells through and facilitates B cell maturation, facilitates T cell maturation
- Secondary lymphoid organs: filter and trap pathogens and foreign substances, filters blood and serves as a reservoir for immune cells, includes tonsils, Peyer's patches, and appendix and provides immune surveillance at mucosal surfaces
- Cells of the immune system
- : B cells produce antibodies and mediate , T cells mediate and assist B cells
- Innate immune cells: phagocytes (, monocytes, ) engulf and destroy pathogens, present to T cells, destroy virus-infected and tumor cells
- : are the most abundant white blood cells and are the first responders to infection, combat parasitic infections and allergic reactions, release histamine and other inflammatory mediators
- Soluble factors
- are signaling molecules that regulate immune cell communication and function (, , )
- proteins are plasma proteins that enhance phagocytosis and induce when activated by pathogens or antibodies
- Lymphoid organs
Innate Immune Recognition
- : conserved molecular structures found on pathogens
- : receptors on immune cells that recognize PAMPs and trigger innate immune responses
- : cell surface proteins that present antigens to T cells, crucial for adaptive immune responses
Types of Immune Responses
Antibody vs cell-mediated immunity
- Antibody-mediated (humoral) immunity
- Mediated by B cells and the antibodies they produce
- Antibodies neutralize and eliminate extracellular pathogens and toxins
- Steps in antibody-mediated immunity
- B cells recognize specific antigens using surface receptors and differentiate into
- Plasma cells secrete large quantities of antigen-specific antibodies into the bloodstream and lymph
- Antibodies bind to pathogens or toxins, marking them for destruction by phagocytes or complement activation
- Antibody functions
- Neutralization: directly bind to and inactivate pathogens (viruses) or toxins (bacterial toxins)
- : coat pathogens to enhance phagocytosis by and neutrophils
- Complement activation: initiate the complement cascade to enhance immune response through opsonization, chemotaxis, and membrane attack complex formation
-
- Mediated by T cells ( and )
- Targets intracellular pathogens such as viruses and some bacteria (Mycobacterium tuberculosis) and abnormal cells such as cancer cells
- Steps in cell-mediated immunity
- such as dendritic cells and macrophages display foreign antigens on their surface using
- T cells recognize the antigen-MHC complex using T cell receptors and become activated
- CD4+ helper T cells secrete cytokines to coordinate the immune response by activating other immune cells
- CD8+ cytotoxic T cells directly kill infected or abnormal cells through the release of cytotoxic granules
- T cell functions
- CD4+ helper T cells
- : activate macrophages and promote cell-mediated immunity against intracellular pathogens
- : stimulate B cell antibody production and promote humoral immunity against extracellular pathogens
- CD8+ cytotoxic T cells
- Release cytotoxic granules containing and to induce apoptosis in target cells
- Secrete cytokines such as to inhibit viral replication and enhance the immune response
- CD4+ helper T cells
Antigen-antibody interactions
- Antigens
- Substances that elicit a specific immune response
- Can be proteins, polysaccharides, or lipids on the surface of pathogens (bacterial capsules) or foreign substances (pollen)
- are specific regions on an antigen recognized by antibodies or T cell receptors
- Antibodies (immunoglobulins)
- Glycoprotein molecules produced by B cells in response to specific antigens
- Structure
- Y-shaped molecules composed of four polypeptide chains (two heavy chains and two light chains)
- Fab (fragment antigen-binding) region contains the variable region that binds to specific antigens
- Fc (fragment crystallizable) region is the constant region that interacts with immune cells and complement proteins
- Classes (isotypes) of antibodies: , , , ,
- Differ in their Fc regions, which determine their functions and locations in the body (IgA in secretions, IgE in allergic reactions)
-
- Specificity: each antibody binds to a specific epitope on an antigen, like a lock and key
- Affinity: strength of the binding between an antibody and its specific epitope, higher affinity leads to stronger binding
- Avidity: overall strength of the interaction between multiple antibodies and antigens, depends on both affinity and the number of binding sites
- Cross-reactivity: when an antibody recognizes similar epitopes on different antigens, can lead to autoimmune reactions
- Antibody effector functions
- Neutralization: antibodies bind to pathogens or toxins, preventing them from entering or damaging cells (neutralizing viral particles)
- Opsonization: antibodies coat pathogens, making them more easily recognized and engulfed by phagocytes (macrophages, neutrophils)
- Complement activation: antibodies bound to pathogens initiate the complement cascade, leading to enhanced phagocytosis, inflammation, and cell lysis
- : antibodies bind to infected or abnormal cells, marking them for destruction by NK cells or macrophages through the release of cytotoxic granules
- : process by which antigen-specific B and T cells are activated and expanded to mount an effective immune response
Key Terms to Review (77)
Acquired immunity: Acquired immunity is the immunity that develops after exposure to a disease-causing agent, either through infection or vaccination. It involves the activation of specific immune cells and the production of antibodies.
Active immunity: Active immunity is the protection against disease through the production of antibodies by the immune system in response to the presence of an antigen. This can occur naturally through infection or artificially through vaccination.
Active Immunity: Active immunity refers to the body's ability to generate its own immune response to a pathogen or foreign substance, providing long-lasting protection against future infections. This process involves the activation and proliferation of the body's own immune cells, such as lymphocytes, to target and eliminate the invading threat.
Adaptive Immunity: Adaptive immunity is the second line of defense in the immune system, involving specialized cells and processes that target specific pathogens or foreign substances. It is a more sophisticated and targeted response compared to innate immunity, and it develops over time through exposure to various antigens.
Allergens: Allergens are substances that can trigger an abnormal immune response in the body, leading to allergy symptoms. They are typically harmless environmental substances that the immune system mistakenly identifies as threats, causing it to release chemicals like histamine to try to expel them from the body.
Antibodies: Antibodies are specialized proteins produced by the immune system to recognize and neutralize foreign substances, such as bacteria, viruses, and toxins. They are a critical component of the adaptive immune response, playing a central role in the body's defense against disease and infection across various topics in pharmacology and immunology.
Antibody-Dependent Cell-Mediated Cytotoxicity (ADCC): Antibody-dependent cell-mediated cytotoxicity (ADCC) is an immune response mechanism in which antibodies bound to target cells stimulate the destruction of those cells by effector cells, such as natural killer (NK) cells. This process involves the recognition of antibody-coated target cells and the subsequent release of cytotoxic granules by the effector cells, leading to the lysis and elimination of the target cells.
Antigen-antibody interactions: Antigen-antibody interactions involve the binding of antibodies to specific antigens, marking them for destruction by the immune system. This interaction is crucial for identifying and neutralizing pathogens.
Antigen-Presenting Cells (APCs): Antigen-presenting cells (APCs) are a group of specialized immune cells that play a crucial role in initiating and regulating the adaptive immune response. These cells are responsible for capturing, processing, and presenting foreign antigens to T lymphocytes, triggering an immune reaction to eliminate the perceived threat.
Antigens: Antigens are any substances that can trigger an immune response by stimulating the production of antibodies. They are typically foreign molecules or structures that the body recognizes as potentially harmful, initiating a cascade of immune reactions to neutralize or eliminate them.
Autoimmune Disease: An autoimmune disease is a condition in which the body's immune system mistakenly attacks and damages its own healthy tissues and cells. This aberrant immune response can lead to a variety of chronic and debilitating health issues, depending on the specific tissues or organs affected.
B cells: B cells, also known as B lymphocytes, are a type of white blood cell that play a crucial role in the adaptive immune response. They are responsible for the production of antibodies, which help neutralize and eliminate pathogens, and also assist in the activation of other immune cells.
B lymphocytes: B lymphocytes, or B cells, are a type of white blood cell essential for the adaptive immune response. They produce antibodies that neutralize pathogens and play a critical role in humoral immunity.
Bacteria: Bacteria are single-celled microorganisms that are ubiquitous in the environment, playing crucial roles in various biological processes. They are prokaryotic organisms, meaning they lack a true nucleus and membrane-bound organelles found in eukaryotic cells. Bacteria are a fundamental component of the immune system's response, as they can serve as both pathogens and beneficial symbionts.
Basophils: Basophils are a type of white blood cell that play a crucial role in the body's immune response. They are the least common type of granulocyte, but their unique characteristics and functions make them an important component of the innate immune system.
Bone Marrow: Bone marrow is the soft, spongy tissue found inside the bones that is responsible for the production of blood cells, including red blood cells, white blood cells, and platelets. It plays a crucial role in the body's immune system and hematopoiesis, the process of blood cell formation.
CD4+ Helper T Cells: CD4+ helper T cells are a type of lymphocyte that play a crucial role in the adaptive immune response. They are called 'helper' T cells because they coordinate and enhance the activities of other immune cells, such as B cells and cytotoxic T cells, to mount an effective immune defense against pathogens.
CD8+ Cytotoxic T Cells: CD8+ cytotoxic T cells are a type of T lymphocyte that plays a crucial role in the adaptive immune response by directly killing infected or cancerous cells. They express the CD8 co-receptor on their surface, which allows them to recognize and bind to major histocompatibility complex (MHC) class I molecules, triggering their cytotoxic function.
Cell-mediated immunity: Cell-mediated immunity is an immune response that does not involve antibodies but instead relies on the activation of T-cells to fight pathogens. It plays a crucial role in defending against intracellular pathogens and cancer cells.
Cell-Mediated Immunity: Cell-mediated immunity is a type of adaptive immune response that involves the direct action of T cells to eliminate pathogens or infected cells. It is a crucial component of the body's defense mechanisms against intracellular infections, cancer, and organ transplant rejection.
Clonal Selection: Clonal selection is a fundamental principle in immunology that describes how the adaptive immune system generates a diverse repertoire of antigen-specific lymphocytes and selectively expands those that can recognize and respond to foreign pathogens. It is a crucial mechanism that underpins the adaptive immune response.
Complement: Complement is a system of proteins found in the blood that work together with the immune system to recognize, opsonize, and eliminate pathogens and damaged cells. It is an important component of the body's innate immune response.
Cytokines: Cytokines are small signaling proteins secreted by various cells in the body, particularly immune cells, that play a crucial role in regulating the immune response, inflammation, and other physiological processes. They act as chemical messengers, allowing cells to communicate with each other and coordinate their activities.
Dendritic Cells: Dendritic cells are a type of antigen-presenting cell that play a crucial role in the initiation and regulation of the immune response. They are found in tissues that are in contact with the external environment, such as the skin, and act as sentinels, constantly sampling their surroundings for potential threats.
Eosinophils: Eosinophils are a type of white blood cell that play a crucial role in the body's immune response, particularly in the context of allergic reactions and parasitic infections. They are characterized by their distinctive bilobed nucleus and bright red-orange granules within their cytoplasm.
Epitopes: Epitopes are the specific regions on the surface of an antigen that are recognized and bound by antibodies or T cell receptors. They are the part of the antigen that the immune system identifies and responds to, initiating an immune response against the target.
Fab Region: The Fab region, or fragment antigen-binding region, is a portion of an antibody molecule that is responsible for recognizing and binding to specific antigens. It is the variable part of the antibody that gives it the ability to recognize a wide range of foreign substances and target them for neutralization or destruction.
Fc Region: The Fc region, or fragment crystallizable region, is a portion of an antibody molecule that does not participate directly in antigen binding. It is the tail end of the antibody that interacts with various immune cells and molecules, playing a crucial role in the immune response.
Granulocytes: Granulocytes are a type of white blood cell that contain granules in their cytoplasm. They are a crucial component of the innate immune system, playing a vital role in the body's immediate response to pathogens and inflammation.
Granzymes: Granzymes are a family of serine proteases found in the cytotoxic granules of natural killer (NK) cells and cytotoxic T lymphocytes (CTLs). These enzymes play a crucial role in the immune system's ability to induce apoptosis, or programmed cell death, in target cells infected with pathogens or transformed into cancerous cells.
Hematopoiesis: Hematopoiesis is the process by which blood cells are formed and developed within the body. It is a crucial component of the immune system, responsible for the continuous production and replenishment of various blood cell types, including red blood cells, white blood cells, and platelets.
Humoral Immunity: Humoral immunity is a key component of the adaptive immune response, involving the production of antibodies by B cells to neutralize and eliminate foreign pathogens and toxins from the body. It is a critical defense mechanism that works in conjunction with the cell-mediated immune response to provide comprehensive protection against a wide range of infectious agents.
IFN-γ: IFN-γ, or interferon-gamma, is a cytokine that plays a crucial role in the immune system's response to pathogens and the regulation of the inflammatory process. As a key signaling molecule, IFN-γ coordinates various immune cells and functions to mount an effective defense against infections and promote tissue repair.
IgA: IgA, or Immunoglobulin A, is a type of antibody that plays a crucial role in the body's mucosal immune system. It is the predominant antibody found in secretions such as saliva, tears, and mucus, where it helps protect against pathogens that may enter the body through these surfaces.
IgD: IgD is an immunoglobulin isotype that is expressed on the surface of mature, naive B cells. It acts as a receptor that helps initiate the humoral immune response by signaling the activation and differentiation of B cells upon antigen binding.
IgE: IgE, or immunoglobulin E, is a class of antibody that plays a crucial role in the body's immune response, particularly in allergic reactions and the defense against parasitic infections. IgE is produced by plasma B cells and binds to specific receptors on the surface of mast cells and basophils, triggering the release of inflammatory mediators like histamine.
IgG: IgG, or Immunoglobulin G, is the most abundant type of antibody found in the human body. It is a critical component of the adaptive immune system, responsible for recognizing and neutralizing foreign pathogens, toxins, and infected cells to protect the body from disease.
IgM: IgM is the largest and first antibody produced by the body's immune system in response to an initial exposure to an antigen. It is a crucial component of the humoral immune response, playing a vital role in the early stages of infection detection and pathogen neutralization.
Immunity: Immunity refers to the body's ability to resist and fight off infections and diseases. It is the system that protects the body from harmful pathogens, such as viruses, bacteria, and toxins, and helps maintain overall health and well-being.
Immunodeficiency: Immunodeficiency refers to a state where the immune system's ability to fight infections and diseases is impaired or weakened, making an individual more susceptible to various types of illnesses and infections. This condition can be either inherited or acquired, and it can have significant implications for an individual's overall health and well-being.
Immunoglobulin: Immunoglobulins, also known as antibodies, are glycoprotein molecules produced by plasma B cells that play a crucial role in the adaptive immune response. They recognize and bind to specific foreign antigens, initiating a series of immune reactions to neutralize or eliminate the target.
Immunological Memory: Immunological memory is the ability of the immune system to mount a faster, more robust response to a pathogen that it has encountered previously. This enhanced response is a hallmark of the adaptive immune system and is crucial for long-term protection against infectious diseases.
Immunosuppressants: Immunosuppressants are a class of drugs that work by reducing or preventing the body's immune response. They are commonly used to prevent rejection of transplanted organs, treat autoimmune disorders, and manage certain inflammatory conditions.
Inflammation: Inflammation is the body's natural immune response to various stimuli, such as pathogens, damaged cells, or irritants. It is a complex biological process that aims to remove harmful stimuli, initiate the healing process, and restore homeostasis. Inflammation is a key component in the understanding of immunity, immunosuppressants, corticosteroids, and other related pharmacological topics.
Innate immune system: The innate immune system is the body's first line of defense against pathogens, providing rapid but non-specific responses. It includes physical barriers, immune cells, and various proteins that identify and eliminate invaders.
Innate Immunity: Innate immunity refers to the body's natural, non-specific defense mechanisms that provide immediate protection against pathogens and foreign substances. It is the first line of defense in the immune response, acting as a barrier to prevent the entry and spread of infectious agents within the body.
Interferons: Interferons are a group of signaling proteins released by host cells in response to the presence of pathogens, such as viruses, bacteria, and parasites. They act as important mediators of the innate immune response, playing a crucial role in the body's defense against infections and the regulation of the immune system.
Interleukins: Interleukins are a group of cytokines, or signaling proteins, that are primarily secreted by white blood cells and play a crucial role in regulating immune and inflammatory responses in the body. They act as chemical messengers, facilitating communication between different cell types and coordinating the complex network of the immune system.
Lymph: Lymph is a clear, colorless fluid that circulates throughout the body's lymphatic system, carrying waste products and foreign materials away from tissues and toward the bloodstream. It plays a crucial role in the body's immune response and overall health.
Lymph Nodes: Lymph nodes are small, bean-shaped organs that are part of the lymphatic system. They act as filters for the lymphatic fluid, trapping bacteria, viruses, and other harmful substances, and play a crucial role in the body's immune response.
Lymphocytes: Lymphocytes are a type of white blood cell that play a crucial role in the body's immune response. They are responsible for recognizing and responding to foreign substances, pathogens, and abnormal cells, making them a vital component of the immune system.
Macrophages: Macrophages are white blood cells within tissues that play a crucial role in the immune system by engulfing and digesting cellular debris, foreign substances, and pathogens. They also stimulate other immune cells by presenting antigens to T cells.
Macrophages: Macrophages are a type of immune cell that play a crucial role in the body's defense against pathogens and in the regulation of the inflammatory response. They are large, phagocytic cells that originate from monocytes and are found in various tissues throughout the body, where they act as the first line of defense against infection and injury.
Major Histocompatibility Complex (MHC): The major histocompatibility complex (MHC) is a group of genes that encode cell surface proteins responsible for the regulation of the immune system. These proteins play a crucial role in the recognition of self from non-self, allowing the body to distinguish its own cells from foreign or infected cells, and mount an appropriate immune response.
Mast cells: Mast cells are a type of white blood cell that play a crucial role in the immune system by releasing histamine and other chemicals during inflammatory and allergic reactions. They are found in connective tissues throughout the body, particularly near blood vessels and nerves.
MHC Molecules: MHC (Major Histocompatibility Complex) molecules are a group of proteins found on the surface of cells that play a crucial role in the immune system's ability to recognize and respond to foreign or abnormal substances within the body. These molecules are essential for the presentation of antigenic peptides to T cells, initiating an immune response.
Mucosa-Associated Lymphoid Tissue (MALT): Mucosa-associated lymphoid tissue (MALT) is a collection of lymphoid follicles and other immune cells that are strategically located within the mucosal linings of various organs and tissues in the body. MALT plays a crucial role in the immune system's defense against pathogens that may enter the body through these mucosal surfaces.
Natural Killer (NK) Cells: Natural killer (NK) cells are a type of lymphocyte that play a crucial role in the innate immune system. They are called 'natural' because they can rapidly respond to and destroy infected or cancerous cells without prior sensitization or activation, unlike other lymphocytes like T cells and B cells.
Neutrophils: Neutrophils are a type of white blood cell crucial for the body's defense against infections. They play a key role in the immune response by targeting and destroying bacteria and fungi.
Neutrophils: Neutrophils are a type of white blood cell that play a crucial role in the body's immune response and inflammatory processes. As the most abundant type of white blood cell, neutrophils are the first line of defense against invading pathogens and are essential for maintaining overall health and well-being.
Opsonization: Opsonization is the process by which certain molecules, called opsonins, bind to the surface of a pathogen or foreign particle, making it more susceptible to phagocytosis and destruction by phagocytic cells, such as macrophages and neutrophils. This process enhances the ability of the immune system to recognize and eliminate these targets.
Passive immunity: Passive immunity is the transfer of active humoral immunity in the form of ready-made antibodies from one individual to another. It provides immediate but short-term protection against infectious agents.
Passive Immunity: Passive immunity is a type of acquired immunity that involves the transfer of antibodies from an external source to an individual, providing them with immediate, short-term protection against a specific pathogen or toxin. This form of immunity is in contrast to active immunity, which is the body's own immune response developed through exposure to an antigen or vaccination.
Pathogen-Associated Molecular Patterns (PAMPs): Pathogen-Associated Molecular Patterns (PAMPs) are conserved molecular structures that are unique to pathogens and are recognized by the innate immune system as a sign of infection. These patterns act as molecular signatures that alert the host's immune defenses to the presence of potentially harmful microorganisms.
Pattern Recognition Receptors (PRRs): Pattern recognition receptors (PRRs) are a class of receptors that recognize specific molecular patterns associated with pathogens or damaged cells, initiating an innate immune response. These receptors act as the first line of defense against infectious agents and cellular stress, playing a crucial role in the body's ability to detect and respond to threats.
Perforin: Perforin is a cytolytic protein produced by cytotoxic T cells and natural killer (NK) cells that plays a crucial role in the immune system's defense against pathogens and infected or cancerous cells. It functions by forming pores in the target cell's membrane, facilitating the entry of other cytotoxic molecules that induce cell death.
Phagocytosis: Phagocytosis is the process by which certain cells, called phagocytes, engulf and digest foreign particles, pathogens, or cellular debris. This process is a crucial component of the body's immune response and plays a vital role in maintaining homeostasis and defending against infections.
Plasma Cells: Plasma cells are terminally differentiated B cells that have a crucial role in the adaptive immune response. They are responsible for the production and secretion of antibodies, which are essential for neutralizing pathogens and marking them for destruction by other immune cells.
Self-Tolerance: Self-tolerance is the ability of the immune system to recognize and not mount an immune response against the body's own healthy cells and tissues. It is a critical mechanism that prevents autoimmune diseases, where the immune system mistakenly attacks the host's own cells.
Spleen: The spleen is a vital organ located in the upper left part of the abdomen, behind the stomach. It plays a crucial role in the body's immune system and serves various functions related to blood filtration, storage, and recycling.
T Cells: T cells, also known as T lymphocytes, are a type of white blood cell that play a central role in the adaptive immune response. They are responsible for cell-mediated immunity, recognizing and responding to specific foreign antigens in the body.
Th1 Cells: Th1 cells, or Type 1 T helper cells, are a subset of CD4+ T cells that play a crucial role in cell-mediated immunity. They are responsible for coordinating the body's immune response against intracellular pathogens, such as viruses and certain bacteria.
Th2 Cells: Th2 cells, or T helper 2 cells, are a subtype of T lymphocytes that play a crucial role in the adaptive immune response. They are primarily responsible for orchestrating the humoral immune response, which involves the production of antibodies by B cells.
Thymus: The thymus is a small, bilobed organ located in the upper chest, just behind the breastbone. It plays a crucial role in the development and maturation of T cells, a type of lymphocyte that is essential for the proper functioning of the immune system.
Tumor Necrosis Factors: Tumor necrosis factors (TNFs) are a group of cytokines, or signaling proteins, that play a crucial role in the immune system's response to inflammation, infection, and cancer. These proteins are involved in regulating cell death, proliferation, and differentiation, making them an important component of the body's defense mechanisms.
Vaccines: Vaccines are biological preparations that provide active acquired immunity to a particular infectious disease. They work by stimulating the body's immune system to recognize and fight off specific pathogens, thereby preventing or reducing the severity of the targeted infection.
Viruses: Viruses are infectious agents that lack the ability to replicate on their own and must hijack the cellular machinery of a host organism to reproduce. They are composed of genetic material, either DNA or RNA, enclosed within a protective protein coat called a capsid.