Humoral Mediated Immunity
Humoral mediated immunity, also known as humoral immunity, is a specific branch of the immune system that involves the production of antibodies in response to foreign substances, known as antigens. These antibodies are produced by specialized white blood cells called B cells. The term “humoral” refers to the fact that this type of immunity is mediated by substances found in the body fluids, particularly the blood and lymph.
When an antigen, such as a bacterium or virus, enters the body, it triggers the activation of B cells. These B cells then undergo a process called clonal selection and differentiation. This leads to the formation of plasma cells, which are highly specialized cells that produce and secrete specific antibodies.
These antibodies circulate in the bloodstream and lymphatic system, where they bind to the corresponding antigens with high specificity. This binding process, known as antigen-antibody interaction, marks the foreign substances for destruction by other components of the immune system, such as phagocytes or complement proteins.
Humoral immunity is especially effective against extracellular pathogens, which are pathogens that exist outside of cells, like bacteria and toxins. It also plays a crucial role in immune responses to certain viruses and in the defense against toxins produced by bacteria.
Humoral Immune Response
- Recognition of Antigen: The process begins when immune cells, such as dendritic cells, present fragments of the foreign antigen to B cells. This interaction activates the B cell.
- Clonal Selection: The activated B cell undergoes clonal selection. This means it starts to divide and produce many identical copies of itself, creating a population of B cells specific to that particular antigen.
- Differentiation into Plasma Cells: Some of the cloned B cells further differentiate into specialized cells called plasma cells. These plasma cells are responsible for producing and secreting antibodies.
- Antibody Production: Each plasma cell produces large quantities of a specific type of antibody. Antibodies are Y-shaped proteins that have a binding site that matches the specific antigen they were activated against.
- Antibody Circulation: The antibodies are released into the bloodstream and lymphatic system. They circulate throughout the body, seeking out and binding to the specific antigens.
- Antigen-Antibody Interaction: When an antibody encounters its corresponding antigen, they bind together in a highly specific manner. This is known as an antigen-antibody interaction.
- Neutralization and Tagging: Antibodies have several functions. They can neutralize pathogens by blocking their ability to infect cells. They can also tag pathogens for destruction by other components of the immune system.
- Memory B Cells: Alongside plasma cells, some B cells become memory B cells. These cells “remember” the specific antigen, providing the immune system with a rapid and heightened response if the same antigen is encountered in the future.
Cell Mediated Immunity
Cell-mediated immunity is a specific branch of the immune system that involves the activation of immune cells, particularly T cells, to directly combat and eliminate infected or abnormal cells in the body. Unlike humoral immunity, which involves the production of antibodies, cell-mediated immunity does not rely on antibodies. Instead, it primarily relies on the activation and function of T cells.
When activated, cytotoxic T cells (also known as killer T cells) directly target and destroy cells that are infected with viruses, intracellular bacteria, or have become cancerous. Additionally, other types of T cells, such as helper T cells, play a vital role in coordinating and regulating immune responses.
Cell-mediated immunity is particularly important in defense against intracellular pathogens (those that live and replicate inside cells), including certain viruses like HIV, and in the detection and elimination of cancer cells. It also plays a crucial role in transplant rejection, as it recognizes and targets foreign tissues.
Cell Mediated immune Response
- Antigen Presentation: Antigen-presenting cells (APCs), such as dendritic cells and macrophages, ingest foreign substances (antigens) and present fragments of these antigens on their surface along with special molecules called major histocompatibility complex (MHC) molecules.
- T Cell Activation: Helper T cells, a type of T cell, recognize the antigen-MHC complex on the surface of APCs. This interaction activates the helper T cell.
- Activation of Cytotoxic T Cells: Once activated, helper T cells release chemical signals (cytokines) that stimulate the activation of cytotoxic T cells. Cytotoxic T cells are specialized cells that directly target and kill infected or abnormal cells.
- Proliferation and Differentiation: Activated cytotoxic T cells undergo clonal selection and differentiation, resulting in a population of cytotoxic T cells specific to the particular antigen.
- Targeting Infected Cells: The activated cytotoxic T cells circulate in the bloodstream and lymphatic system, seeking out cells that have been infected by pathogens, such as viruses.
- Recognition and Destruction: When a cytotoxic T cell encounters a cell with the specific antigen, it binds to it through the antigen-MHC complex. The cytotoxic T cell then releases enzymes that induce apoptosis (programmed cell death) in the infected cell.
- Memory T Cells: Alongside the cytotoxic T cells, some T cells become memory T cells. These cells “remember” the specific antigen, providing the immune system with a rapid and heightened response if the same antigen is encountered in the future.
Types of Cell mediated immunity on the basis of Effector response:
- Cytotoxic (Cell-Killing) T Cell Response:
- Effector Cells: Cytotoxic T cells (also known as CD8+ T cells) are the primary effector cells in this type of cell-mediated immunity.
- Effector Response: Cytotoxic T cells directly target and kill infected or abnormal cells. They recognize cells that display foreign antigens on their surface, usually due to viral infection or cancerous transformation. Once activated, cytotoxic T cells induce apoptosis (programmed cell death) in the targeted cell.
- Helper T Cell Response:
- Effector Cells: Helper T cells (also known as CD4+ T cells) are the primary effector cells in this type of cell-mediated immunity.
- Effector Response: Helper T cells do not directly kill infected cells. Instead, they play a coordinating and regulatory role in the immune response. They release signaling molecules called cytokines, which help activate and direct other immune cells. Helper T cells are crucial for the activation of cytotoxic T cells, B cells (for antibody production), and macrophages.
Stages in Antigen specific Cell mediated killing of target cells
- Antigen Presentation: The process begins when antigen-presenting cells (APCs), such as dendritic cells or macrophages, encounter and ingest foreign antigens, often from pathogens like viruses. The APC processes the antigen and presents fragments of it on its surface bound to major histocompatibility complex (MHC) molecules.
- T Cell Activation: A specific cytotoxic T cell (CD8+ T cell) recognizes the antigen-MHC complex presented by the APC. This recognition triggers the activation of the cytotoxic T cell.
- Clonal Selection and Expansion: The activated cytotoxic T cell undergoes clonal selection and starts to divide, producing many identical copies of itself. This leads to the formation of a population of cytotoxic T cells specific to the particular antigen.
- Migration to Target Site: The activated cytotoxic T cells, now in a larger population, migrate through the bloodstream and lymphatic system to reach the site where the infected or abnormal cells are located.
- Recognition of Target Cells: The cytotoxic T cells search for cells that display the specific antigen on their surface, indicating that they are infected or abnormal. These cells are typically recognized due to the presence of foreign proteins, such as viral proteins, on their surface.
- Binding and Activation: When a cytotoxic T cell encounters a target cell displaying the specific antigen, it binds to the cell through the interaction of its T cell receptor (TCR) and the antigen-MHC complex on the surface of the target cell.
- Release of Cytotoxic Granules: The cytotoxic T cell releases specialized vesicles called cytotoxic granules. These granules contain perforin and granzymes.
- Perforin: Perforin creates pores in the target cell’s membrane, allowing granzymes to enter.
- Granzymes: Granzymes are enzymes that initiate apoptosis (programmed cell death) within the target cell.
- Induction of Apoptosis: Granzymes enter the target cell and trigger a cascade of events that lead to apoptosis. This controlled cell death ensures that the infected or abnormal cell is efficiently eliminated without causing inflammation or damage to neighboring cells.
- Destruction of Target Cell: The target cell undergoes apoptosis, breaking down into fragments that can be easily cleared by the immune system.
- Memory T Cells Formation: Alongside the cytotoxic T cells, some T cells become memory T cells. These cells “remember” the specific antigen, providing the immune system with a rapid and heightened response if the same antigen is encountered in the future.
Important Differences between Humoral Mediated Immunity and Cell Mediated Immunity
Basis of Comparison
|Humoral Mediated Immunity||Cell Mediated Immunity|
|Main Effector Cells||B cells (and their products, antibodies)||T cells (Cytotoxic T cells and Helper T cells)|
|Primary Target||Extracellular pathogens (outside of cells)||Intracellular pathogens (inside cells) and abnormal cells (e.g., cancerous cells)|
|Response to Antigens||Responds to freely circulating antigens in body fluids||Responds to antigens presented on the surface of cells|
|Effector Molecules||Antibodies (immunoglobulins) are the main effector molecules||Cytokines (chemical signaling molecules) and cytotoxic substances (e.g., perforin, granzymes)|
|Direct Cell Killing||Not involved in direct killing of infected cells||Involves direct killing of infected or abnormal cells|
|Activation of B or T Cells||B cells are activated when they encounter antigens and differentiate into plasma cells||T cells are activated after antigen presentation, leading to clonal expansion and differentiation|
|Primary Defense Against||Bacterial infections and toxins, extracellular viruses, and foreign substances in body fluids||Viral infections, intracellular bacteria, cancer cells, and cells infected with intracellular pathogens|
|Role in Transplant Rejection||Minimal role in transplant rejection||Plays a significant role in transplant rejection by recognizing foreign tissue|
|Memory Response||Involves memory B cells for rapid antibody production upon re-exposure||Involves memory T cells for a rapid and heightened response upon re-exposure|
|Examples of Immune Responses||Response to bacterial infections, neutralizing toxins, and allergic reactions||Response to viral infections, intracellular pathogens, and immune surveillance for cancer|
|Primary Mediators||Antibodies mediate the immune response||T cells, particularly cytotoxic T cells and helper T cells, mediate the response|
|Protection at Mucosal Surfaces||Provides protection at mucosal surfaces due to the presence of secretory antibodies (IgA)||Offers limited protection at mucosal surfaces|
|Role in Passive Immunity||Antibodies can be transferred passively (e.g., through breast milk or immunoglobulin therapy)||T cells do not play a significant role in passive immunity|
Similarities between Humoral Mediated Immunity and Cell Mediated Immunity
- Both Are Adaptive Immune Responses: Both humoral and cell-mediated immunity are part of the adaptive immune system. This means that they are highly specific and targeted, responding to particular antigens.
- Dependence on Lymphocytes: Both responses rely on the activation and differentiation of lymphocytes, a type of white blood cell. Humoral immunity involves B cells, while cell-mediated immunity involves T cells.
- Antigen Specificity: In both types of immunity, the response is triggered by the presence of specific antigens. These antigens are usually foreign substances or molecules associated with pathogens.
- Memory Response: Both humoral and cell-mediated immunity generate memory cells. These memory cells “remember” specific antigens, allowing for a more rapid and robust response upon re-exposure to the same antigen.
- Effector Phase: Both responses have an effector phase where activated lymphocytes carry out their functions. In humoral immunity, this involves the production of antibodies by plasma cells. In cell-mediated immunity, it involves the activation of cytotoxic T cells to directly target and kill infected or abnormal cells.
- Coordination in Immune Responses: Both types of immunity work together in the immune system to provide comprehensive protection. Humoral immunity is particularly effective against extracellular pathogens, while cell-mediated immunity is crucial for intracellular pathogens and abnormal cells.
- Protection Against Different Pathogens: Together, they provide a multi-faceted defense against a wide range of pathogens, including bacteria, viruses, fungi, and cancer cells.
- Role in Immunological Memory: Both contribute to the establishment of immunological memory. This memory ensures that the immune system can mount a faster and more effective response upon encountering a previously encountered antigen.
- Can Be Activated Simultaneously: In many infections, both humoral and cell-mediated immune responses are activated simultaneously, working in concert to combat the invading pathogens.
- Can Contribute to Inflammation: Both types of immunity can lead to inflammatory responses as part of their efforts to eliminate pathogens and infected cells.
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