12.4 Cell-mediated immunity
5 min read•august 14, 2024
Cell-mediated immunity is a crucial part of our immune system's defense against pathogens. , or , are the key players in this process, recognizing and targeting infected or abnormal cells within the body.
T cells come in different types, each with specific jobs. They develop in the thymus, where they learn to recognize foreign invaders without attacking our own cells. This careful selection process ensures our immune system protects us without causing harm.
T Lymphocytes in Cell-Mediated Immunity
Types and Functions of T Lymphocytes
Top images from around the web for Types and Functions of T Lymphocytes
T cell - Wikipedia View original
Is this image relevant?
The Adaptive Immune Response: T lymphocytes and Their Functional Types · Anatomy and Physiology View original
Is this image relevant?
The Adaptive Immune Response: T lymphocytes and Their Functional Types | Anatomy and Physiology II View original
Is this image relevant?
T cell - Wikipedia View original
Is this image relevant?
The Adaptive Immune Response: T lymphocytes and Their Functional Types · Anatomy and Physiology View original
Is this image relevant?
1 of 3
Top images from around the web for Types and Functions of T Lymphocytes
T cell - Wikipedia View original
Is this image relevant?
The Adaptive Immune Response: T lymphocytes and Their Functional Types · Anatomy and Physiology View original
Is this image relevant?
The Adaptive Immune Response: T lymphocytes and Their Functional Types | Anatomy and Physiology II View original
Is this image relevant?
T cell - Wikipedia View original
Is this image relevant?
The Adaptive Immune Response: T lymphocytes and Their Functional Types · Anatomy and Physiology View original
Is this image relevant?
1 of 3
- T lymphocytes, or T cells, are a type of white blood cell that plays a central role in cell-mediated immunity, which is the immune response that targets infected or abnormal cells within the body
- T cells originate from the bone marrow and mature in the thymus, where they undergo a selection process to ensure they can recognize foreign antigens presented by the body's own cells
- There are several subsets of T cells, each with specific functions in the cell-mediated immune response:
- () directly kill infected, damaged, or cancerous cells by releasing cytotoxic granules containing and
- () secrete that stimulate and coordinate the activities of other immune cells, such as B cells and macrophages
- () help maintain self-tolerance and prevent autoimmune responses by suppressing the activity of other T cells
- are long-lived cells that persist after an infection is cleared, allowing for a faster and stronger immune response upon re-exposure to the same pathogen (influenza virus)
Development and Selection of T Cells
- T cells develop from hematopoietic stem cells in the bone marrow and migrate to the thymus for maturation
- In the thymus, T cells undergo , ensuring they can recognize self-MHC molecules, and , eliminating self-reactive T cells that could cause autoimmunity
- Positive selection occurs in the thymic cortex, where T cells that bind to self-MHC molecules with moderate affinity receive survival signals and continue to mature
- Negative selection takes place in the thymic medulla, where T cells that bind too strongly to self-antigens presented by medullary thymic epithelial cells (mTECs) undergo , preventing autoimmunity
- The surviving T cells then differentiate into either CD4+ or CD8+ T cells, depending on their affinity for or molecules, respectively
T Cell Activation and Antigen Presentation
Antigen Recognition and Costimulation
- T cell activation requires two main signals: antigen recognition through the (TCR) and provided by antigen-presenting cells (APCs)
- Antigens are presented to T cells by major histocompatibility complex (MHC) molecules on the surface of APCs, such as dendritic cells, macrophages, and B cells
- MHC class I molecules present intracellular antigens (viral proteins) to CD8+ T cells, while MHC class II molecules present extracellular antigens (bacterial proteins) to CD4+ T cells
- The TCR on the T cell surface recognizes and binds to the antigen-MHC complex, providing the first signal for T cell activation
- Costimulatory molecules, such as CD28 on T cells and B7 on APCs, interact to provide the second signal necessary for full T cell activation and prevent anergy (unresponsiveness)
Clonal Expansion and Differentiation
- Upon activation, T cells undergo , rapidly dividing to produce a large number of antigen-specific
- Activated T cells also differentiate into various subsets, such as cytotoxic T cells, helper T cells (Th1, Th2, Th17), and regulatory T cells, each with specific functions in the immune response
- Some activated T cells differentiate into memory T cells, which persist long after the initial infection is cleared and provide rapid and enhanced protection upon re-exposure to the same pathogen (measles virus)
- The expansion and differentiation of T cells is driven by cytokines, such as , which promotes T cell proliferation and survival
Effector Mechanisms of Cell-Mediated Immunity
Cytotoxic T Cell Responses
- Cytotoxic T cells (CTLs) directly kill target cells infected with intracellular pathogens (viruses) or displaying abnormal antigens (tumor cells)
- CTLs release cytotoxic granules containing perforin, which forms pores in the target cell membrane, and granzymes, which induce apoptosis (programmed cell death) in the target cell
- CTLs can also induce apoptosis in target cells through the , which triggers a leading to cell death
- CTLs play a crucial role in eliminating virus-infected cells and controlling viral infections, such as influenza and HIV
- CTLs also contribute to anti-tumor immunity by recognizing and killing cancer cells that express ()
Activation of Macrophages and Phagocytic Cells
- Helper T cells (Th cells) secrete cytokines that activate and enhance the functions of macrophages and other , enabling them to more effectively eliminate extracellular pathogens (bacteria)
- produce (), which activates macrophages, increasing their phagocytic and microbicidal activities
- Activated macrophages also secrete , such as and , which further stimulate the cell-mediated immune response
- Th cells also provide help to B cells, promoting antibody production and class switching, which contributes to the humoral immune response against extracellular pathogens
- The interaction between Th cells and macrophages is essential for the control of intracellular bacterial infections, such as tuberculosis and listeriosis
T Cell Regulation of Immune Response
Helper T Cell Subsets and Functions
- T cells play a crucial role in regulating the immune response, ensuring that it is effective against pathogens while preventing excessive inflammation and autoimmunity
- Helper T cells (CD4+ T cells) regulate the immune response by secreting cytokines that modulate the activities of other immune cells
- Th1 cells produce IFN-γ and IL-2, which promote cell-mediated immunity by activating macrophages and stimulating the proliferation of cytotoxic T cells
- secrete , , and , which stimulate B cell antibody production and class switching, supporting humoral immunity
- produce and , which recruit neutrophils and promote mucosal immunity against extracellular bacteria and fungi (Candida albicans)
- The balance between Th1 and Th2 responses is critical for the appropriate immune response against different types of pathogens and in preventing allergic reactions
Regulatory T Cells and Immune Tolerance
- Regulatory T cells (Tregs) maintain self-tolerance and prevent autoimmune responses by suppressing the activity of self-reactive T cells and other immune cells
- Tregs express the transcription factor and secrete immunosuppressive cytokines, such as IL-10 and , which inhibit T cell proliferation and effector functions
- Tregs also express CTLA-4, a negative regulator of T cell activation, which competes with CD28 for binding to B7 on APCs, thereby limiting T cell activation
- Tregs play a crucial role in preventing autoimmune diseases, such as type 1 diabetes and multiple sclerosis, by suppressing the activation and expansion of self-reactive T cells
- Dysregulation of T cell-mediated immune responses can lead to chronic inflammation, autoimmune diseases, and immunodeficiencies, highlighting the importance of proper T cell function in maintaining immune homeostasis
Key Terms to Review (45)
Antigen presentation: Antigen presentation is the process by which immune cells display antigenic peptides on their surface using major histocompatibility complex (MHC) molecules. This process is crucial for the activation of T cells, which play a vital role in the immune response. Antigen presentation allows the immune system to recognize foreign pathogens and initiate appropriate responses, linking innate and adaptive immunity together.
Apoptosis: Apoptosis is a programmed cell death process that allows cells to self-destruct in a controlled manner, helping maintain tissue homeostasis and preventing the proliferation of damaged or unwanted cells. This mechanism is crucial for normal development, immune response, and the aging process, linking it to various biological functions such as cell signaling, division, and immune system regulation.
Caspase cascade: The caspase cascade is a series of biochemical events that lead to programmed cell death, or apoptosis, involving the activation of caspases, which are cysteine proteases. This cascade is crucial in the immune response, as it helps eliminate infected or damaged cells, contributing to cell-mediated immunity by regulating the lifespan and function of immune cells.
Cd4+ t cells: CD4+ T cells are a type of white blood cell that plays a crucial role in the immune system by helping to orchestrate the body's response to infections and diseases. They are primarily involved in recognizing foreign antigens presented by antigen-presenting cells and activating other immune cells, such as B cells and cytotoxic T cells, which are essential for effective immune defense. CD4+ T cells are integral to cell-mediated immunity and adaptive immune responses, making them vital for maintaining immune homeostasis.
CD8+ T Cells: CD8+ T cells are a type of immune cell that play a crucial role in cell-mediated immunity by directly killing infected or cancerous cells. They recognize antigens presented by Major Histocompatibility Complex (MHC) class I molecules, which are found on almost all nucleated cells, allowing them to identify and destroy cells that display foreign or abnormal proteins. This targeted action is essential for controlling viral infections and eliminating tumors.
Clonal expansion: Clonal expansion is the process by which specific immune cells, particularly lymphocytes, rapidly multiply in response to encountering an antigen. This mechanism ensures that the body can generate a large population of cells capable of recognizing and attacking the same pathogen, which plays a crucial role in adaptive immunity and the body's defense against infections.
Costimulatory signals: Costimulatory signals are essential molecular interactions that provide a second layer of activation required for T cells to become fully activated upon encountering an antigen. These signals occur alongside the primary signal from the interaction between T cell receptors and antigens presented by major histocompatibility complex (MHC) molecules. The presence of costimulatory signals is crucial for T cell proliferation, differentiation, and the subsequent immune response.
Cytokines: Cytokines are small signaling proteins that play a crucial role in cell communication and coordination during immune responses. They are produced by various cells in the immune system and act on other cells to modulate inflammation, immune responses, and hematopoiesis. Their ability to influence both innate and adaptive immunity highlights their importance in maintaining homeostasis and orchestrating complex immune reactions.
Cytotoxic T cells: Cytotoxic T cells are a type of immune cell that play a crucial role in the body's defense against infections and cancer. They recognize and destroy infected or malignant cells by releasing cytotoxic substances, which induce apoptosis, or programmed cell death. These cells are essential for cell-mediated immunity, acting as the body's 'killer' cells to eliminate threats effectively.
Effector T cells: Effector T cells are specialized immune cells that arise from activated T lymphocytes and are crucial for the adaptive immune response. These cells play a significant role in identifying and eliminating infected or malignant cells, primarily through the release of cytokines and direct cytotoxic activity. Their activation leads to a robust immune response that is tailored to specific pathogens or abnormalities.
Fas-fas ligand pathway: The fas-fas ligand pathway is a crucial mechanism in the immune system that facilitates apoptosis, or programmed cell death, by interacting between fas receptors on target cells and fas ligands on effector cells. This interaction is significant for regulating immune responses, maintaining homeostasis, and eliminating cells that are damaged or infected. By mediating apoptosis, the pathway plays a vital role in preventing autoimmune diseases and controlling the growth of tumors.
Foxp3: Foxp3 is a transcription factor that plays a critical role in the development and function of regulatory T cells (Tregs), which are essential for maintaining immune tolerance and preventing autoimmune diseases. This protein ensures that Tregs can effectively suppress unwanted immune responses, thus protecting the body from excessive inflammation and autoimmunity. In the context of cell-mediated immunity, Foxp3 is key in regulating the balance between activation and inhibition of T cells.
Granzymes: Granzymes are a family of serine proteases that play a crucial role in the process of programmed cell death, specifically through apoptosis. They are produced by cytotoxic T cells and natural killer (NK) cells, which release them upon recognizing and targeting infected or cancerous cells. By activating a cascade of cellular events leading to apoptosis, granzymes help maintain the body's immune response and eliminate harmful cells.
Helper T cells: Helper T cells are a subset of T lymphocytes that play a crucial role in the immune response by activating and coordinating other immune cells. These cells are essential for both cell-mediated immunity and humoral immunity, as they help activate B cells to produce antibodies and stimulate cytotoxic T cells to destroy infected or cancerous cells. Without helper T cells, the immune system would struggle to mount an effective response against pathogens.
Ifn-γ: IFN-γ, or interferon-gamma, is a crucial cytokine produced primarily by T cells and natural killer (NK) cells that plays a vital role in the immune response. It enhances the ability of immune cells to combat infections and tumors by activating macrophages, promoting antigen presentation, and regulating the expression of major histocompatibility complex (MHC) molecules. Its role is central to cell-mediated immunity, where it helps coordinate the body's defense against intracellular pathogens and supports the activation of cytotoxic T lymphocytes.
IL-12: IL-12, or Interleukin-12, is a cytokine that plays a critical role in the immune system by promoting the differentiation of T cells and enhancing the activity of natural killer (NK) cells. It is produced mainly by macrophages and dendritic cells, influencing the development of cell-mediated immunity by encouraging T cells to become Th1 cells, which are crucial for combating intracellular pathogens. This cytokine also helps orchestrate the adaptive immune response, making it vital for effective defense against infections.
Il-13: IL-13, or Interleukin-13, is a cytokine produced primarily by T-helper 2 (Th2) cells that plays a crucial role in the immune response, particularly in mediating allergic reactions and asthma. This signaling molecule influences various immune processes, including promoting B-cell differentiation and enhancing the production of IgE antibodies, which are often associated with allergic responses. IL-13 is essential for regulating inflammation and can contribute to tissue remodeling in chronic conditions.
Il-17: IL-17, or Interleukin 17, is a pro-inflammatory cytokine produced mainly by a subset of T cells known as Th17 cells. It plays a crucial role in the immune response by promoting inflammation and recruiting immune cells to sites of infection or injury. IL-17 is essential for protecting against extracellular pathogens but can also contribute to the pathology of autoimmune diseases due to its potent inflammatory effects.
IL-2: Interleukin-2 (IL-2) is a cytokine that plays a critical role in the immune system, primarily in the proliferation and activation of T cells. It is produced mainly by activated T cells and stimulates the growth of other immune cells, helping to regulate the immune response. This makes IL-2 essential for cell-mediated immunity, where T cells identify and eliminate infected or cancerous cells.
IL-22: IL-22, or Interleukin-22, is a cytokine produced primarily by T-helper 17 (Th17) cells and is crucial for mediating immune responses and tissue repair. It plays a significant role in cell-mediated immunity by promoting the survival and function of epithelial cells, enhancing their ability to produce antimicrobial peptides, and facilitating the repair of damaged tissues. IL-22 also contributes to inflammation and has been implicated in various autoimmune diseases and chronic inflammatory conditions.
IL-4: IL-4, or Interleukin-4, is a cytokine primarily produced by T-helper 2 (Th2) cells, which plays a crucial role in regulating immune responses and promoting the differentiation of naive T cells into Th2 cells. This cytokine is essential for the development of a specific type of immune response that targets extracellular pathogens and is also involved in the activation of B cells, leading to antibody production, particularly IgE. The significance of IL-4 extends to its involvement in allergic reactions and asthma, highlighting its impact on both normal immune function and pathological conditions.
IL-5: IL-5, or Interleukin 5, is a cytokine that plays a critical role in the regulation and activation of eosinophils, a type of white blood cell involved in the immune response, particularly in allergic reactions and asthma. It is produced mainly by Th2 cells and is essential for the growth, differentiation, and activation of eosinophils, influencing both the innate and adaptive immune responses. Understanding IL-5's function helps in comprehending how immune responses are orchestrated in conditions like asthma and other allergic diseases.
Interferon-gamma: Interferon-gamma is a cytokine produced primarily by T cells and natural killer (NK) cells that plays a critical role in the immune response, particularly in cell-mediated immunity. It enhances the ability of immune cells to respond to pathogens and promotes the activation of macrophages, thereby increasing their capacity to eliminate intracellular infections. Its influence is essential for the adaptive immune system and helps coordinate the immune response against viral and certain bacterial infections.
Memory T cells: Memory T cells are a specialized subset of T lymphocytes that remain in the body after an initial immune response, providing long-term immunity by quickly recognizing and responding to previously encountered antigens. These cells play a crucial role in the adaptive immune system, ensuring a faster and more effective response during subsequent encounters with the same pathogen.
MHC Class I: MHC Class I molecules are proteins located on the surface of almost all nucleated cells that present endogenous antigens to CD8+ T cells, playing a crucial role in cell-mediated immunity. They help the immune system recognize and eliminate infected or abnormal cells by displaying peptide fragments derived from proteins synthesized within the cell, thus alerting cytotoxic T cells to potential threats.
MHC Class II: MHC Class II molecules are proteins found on the surface of certain immune cells that present processed antigens to CD4+ T helper cells, playing a crucial role in initiating immune responses. These molecules are essential for the activation of helper T cells, which subsequently assist in orchestrating various aspects of the adaptive immune response, including the activation of B cells and cytotoxic T cells.
Negative selection: Negative selection is a critical process in the development of T cells, specifically in the thymus, that eliminates those cells which recognize self-antigens too strongly. This mechanism ensures that only T cells that are moderately responsive to foreign antigens are allowed to mature and enter the peripheral immune system. By removing self-reactive T cells, negative selection plays a vital role in preventing autoimmune diseases and maintaining immune tolerance.
Perforin: Perforin is a cytolytic protein that plays a crucial role in the immune response, specifically in cell-mediated immunity. It is secreted by cytotoxic T cells and natural killer (NK) cells to create pores in the membranes of target cells, leading to their destruction. This mechanism is essential for eliminating infected or cancerous cells, ensuring the body's ability to combat pathogens and malignancies effectively.
Phagocytic cells: Phagocytic cells are specialized immune cells that play a crucial role in the body's defense by engulfing and digesting pathogens, dead cells, and debris. These cells are essential for both innate immunity and the activation of adaptive immunity, as they help eliminate harmful invaders and present antigens to other immune cells.
Positive Selection: Positive selection is a process in immunology where T cells that strongly recognize self-MHC (Major Histocompatibility Complex) molecules, presenting foreign antigens, are preferentially selected to survive and proliferate. This ensures that the immune system effectively targets and eliminates pathogens while maintaining tolerance to self-antigens, preventing autoimmune responses.
Pro-inflammatory cytokines: Pro-inflammatory cytokines are signaling molecules released by immune cells that promote inflammation in response to pathogens, tissue injury, or immune activation. These cytokines play a crucial role in cell-mediated immunity by facilitating the recruitment and activation of immune cells to sites of infection or damage, thereby enhancing the body's inflammatory response to combat disease and restore homeostasis.
Regulatory t cell functions: Regulatory T cell functions refer to the roles played by a subset of T cells that help maintain immune system homeostasis by suppressing excessive immune responses and preventing autoimmune diseases. These cells, often referred to as Tregs, play a crucial role in balancing immune activation and tolerance, ensuring that the body's immune responses are appropriately controlled and do not harm the host.
Regulatory T cells: Regulatory T cells, often abbreviated as Tregs, are a specialized subset of T lymphocytes that play a crucial role in maintaining immune tolerance and preventing autoimmunity. They help regulate the immune response by inhibiting the activity of other immune cells, ensuring that the body can distinguish between self and non-self antigens, which is essential for a balanced immune system.
T cell receptor: The T cell receptor (TCR) is a specialized protein found on the surface of T cells that recognizes and binds to specific antigens presented by major histocompatibility complex (MHC) molecules on the surfaces of other cells. This interaction is crucial for the activation and regulation of T cells during immune responses, particularly in cell-mediated immunity where T cells target infected or abnormal cells directly.
T cells: T cells are a type of white blood cell that play a crucial role in the adaptive immune response, specifically in recognizing and eliminating infected or cancerous cells. They are vital for orchestrating the immune system's response and are derived from stem cells in the bone marrow, maturing in the thymus. T cells help to distinguish between self and non-self, making them essential for targeted immunity and overall immune system regulation.
T lymphocytes: T lymphocytes, also known as T cells, are a type of white blood cell that play a crucial role in the immune response, particularly in cell-mediated immunity. These cells originate in the bone marrow but mature in the thymus, where they develop specific receptors that allow them to recognize and respond to antigens presented by infected or abnormal cells. T lymphocytes can be classified into various subsets, including helper T cells and cytotoxic T cells, each with distinct functions in regulating and executing immune responses.
Taas: Taas refers to the T-cell receptor-associated protein that plays a crucial role in the activation and function of T-cells, which are essential components of the immune system. This protein is involved in the signaling pathways that enable T-cells to respond to pathogens and infected cells, ultimately leading to the adaptive immune response. Taas interacts with T-cell receptors (TCRs) to facilitate effective communication within the immune system and contributes to cell-mediated immunity.
Tgf-β: Transforming growth factor beta (tgf-β) is a multifunctional cytokine that plays a crucial role in regulating immune responses, cell growth, and differentiation. It is particularly important in the context of cell-mediated immunity as it can influence T cell development and function, as well as the activity of various immune cells, promoting a balance between immune activation and tolerance.
Th1 cells: Th1 cells, or T helper 1 cells, are a subtype of CD4+ T lymphocytes that play a crucial role in cell-mediated immunity. They are primarily involved in the immune response against intracellular pathogens, such as viruses and some bacteria, by promoting the activation of macrophages and the production of specific cytokines that enhance cellular immune functions. Th1 cells help in coordinating the immune response and are essential for defending against infections.
Th17 cells: Th17 cells are a subset of CD4+ T helper cells that play a crucial role in the immune response by producing pro-inflammatory cytokines, particularly interleukin-17 (IL-17). These cells are essential for defending against extracellular pathogens, such as bacteria and fungi, and are involved in the pathogenesis of various autoimmune diseases. Their distinct cytokine profile and ability to recruit other immune cells make them key players in both protective immunity and inflammatory responses.
Th2 cells: Th2 cells, or T helper 2 cells, are a subset of CD4+ T cells that play a vital role in the immune system by promoting antibody production and orchestrating the response to allergens and parasitic infections. These cells are crucial for activating B cells to produce immunoglobulins, particularly IgE, which is important in defending against parasites and mediating allergic responses. The activation of Th2 cells typically occurs through antigen presentation by dendritic cells and subsequent signaling from cytokines.
Tnf-α: TNF-α, or Tumor Necrosis Factor-alpha, is a cytokine that plays a crucial role in regulating immune responses, inflammation, and cell death. It is primarily produced by activated macrophages and is pivotal in the process of cell-mediated immunity by influencing the activation of T-cells and promoting the inflammatory response against pathogens. This cytokine is also involved in various pathological conditions, making it a key player in both protective immunity and inflammatory diseases.
Tregs: Tregs, or regulatory T cells, are a specialized subset of T cells that play a crucial role in maintaining immune tolerance and preventing autoimmune responses. They help to regulate the immune system by suppressing excessive immune reactions, ensuring that the body can distinguish between self and non-self. This is essential for protecting tissues from damage caused by an overactive immune response while still allowing the body to respond effectively to infections.
Tumor necrosis factor: Tumor necrosis factor (TNF) is a cytokine that plays a crucial role in the inflammatory response and immune system regulation. It is primarily produced by activated macrophages and T cells, and it can induce cell death, promote inflammation, and influence the behavior of immune cells. This cytokine is vital for the defense against infections and tumors, but it can also contribute to autoimmune diseases and chronic inflammatory conditions.
Tumor-associated antigens: Tumor-associated antigens are abnormal proteins expressed on the surface of cancer cells that can trigger an immune response. These antigens differ from normal proteins found in healthy cells, making them recognizable to the immune system, particularly by T cells. They play a crucial role in cell-mediated immunity by helping the body identify and attack tumor cells while leaving normal cells unharmed.