1.4 Cellular components of innate immunity
3 min read•july 25, 2024
Innate immunity is your body's first line of defense against invaders. It's like having a team of specialized bouncers ready to kick out unwanted guests. These cellular defenders work together to quickly identify and neutralize threats before they can cause harm.
Each cell type in innate immunity has a unique role. are the cleanup crew, are rapid responders, and are assassins. bridge innate and adaptive immunity, while and handle specific threats like parasites and allergens.
Cellular Components of Innate Immunity
Cell types in innate immunity
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- Macrophages engulf pathogens through present antigens to T cells produce for immune regulation (TNF-α, IL-1β)
- Neutrophils quickly respond to infections phagocytose pathogens release granules with antimicrobial substances (, )
- Natural Killer (NK) cells recognize and destroy virus-infected cells produce cytokines like IFN-γ to activate other immune cells
- Dendritic cells capture antigens in tissues migrate to lymph nodes present antigens to T cells bridging innate and adaptive immunity
- Mast cells release inflammatory mediators (histamine, leukotrienes) play crucial role in allergic responses and protection against parasites
- Eosinophils defend against parasitic infections (helminths) contribute to allergic reactions through release of cytotoxic granule proteins
Role of macrophages
- Tissue-resident sentinels distributed in various organs (liver, lungs, brain) act as first line of defense against invading pathogens
- Phagocytosis involves engulfment of pathogens and cellular debris formation of phagosomes that fuse with lysosomes for pathogen destruction
- detect activate signaling cascades for immune response
- Activation states include M1 (pro-inflammatory) and M2 (tissue repair) polarization depending on environmental stimuli
- Cytokine production encompasses pro-inflammatory (TNF-α, IL-1β) and immunoregulatory (IL-10, TGF-β) factors shaping immune responses
- Antigen presentation involves processing engulfed pathogens expressing antigens on MHC class II molecules for T cell recognition
Functions of neutrophils
- Rapid recruitment to infection sites guided by extravasate through blood vessel walls following inflammatory signals
- Phagocytosis of pathogens entails engulfment destruction of microorganisms within phagolysosomes
- releases antimicrobial proteins and enzymes (myeloperoxidase, defensins, cathepsins) into extracellular space
- consist of released DNA and histones trap and kill pathogens outside the cell
- Short lifespan ensures rapid turnover during acute inflammation cleared by macrophages after pathogen elimination
- Oxidative burst produces kills engulfed pathogens through oxidative damage
Natural killer cells in immunity
- Recognition of abnormal cells detects altered MHC class I expression identifies stress-induced ligands on infected or transformed cells
- Cytotoxic activity involves release of and induces apoptosis in target cells
- recognizes antibody-coated targets activates NK cells through Fc receptors
- Cytokine production includes IFN-γ for macrophage activation TNF-α for inflammation and cell death
- regulate NK cell activity balance activating and inhibitory signals
- enable enhanced responses to subsequent encounters contribute to trained immunity concept
Dendritic cells as immune links
- Antigen capture and processing involves of pathogens processing antigens for presentation to T cells
- Migration to lymphoid organs occurs after maturation upregulation of chemokine receptors for transport to T cell-rich areas
- Antigen presentation utilizes MHC class I and II molecules displays peptides for T cell recognition and activation
- Costimulatory molecule expression includes CD80, CD86 for T cell activation CD40 for interaction with helper T cells
- Cytokine production encompasses IL-12 for Th1 differentiation IL-23 for Th17 differentiation shaping adaptive responses
- allows presentation of exogenous antigens on MHC class I activates CD8+ T cells against intracellular pathogens
- involves presentation of self-antigens in steady-state maintains peripheral tolerance preventing autoimmunity
Key Terms to Review (27)
Antibody-dependent cell-mediated cytotoxicity (ADCC): Antibody-dependent cell-mediated cytotoxicity (ADCC) is an immune response where antibodies bind to target cells, marking them for destruction by immune cells such as natural killer (NK) cells and macrophages. This process highlights the interaction between the adaptive immune system, through antibodies, and innate immune cells, allowing for the targeted elimination of infected or cancerous cells.
Chemotaxis: Chemotaxis is the movement of cells towards or away from a chemical stimulus, often used by immune cells to locate sites of infection or inflammation. This process is crucial for the functioning of the immune system, as it directs leukocytes to areas where they are needed most to fight off pathogens and initiate repair processes.
Costimulatory Molecules: Costimulatory molecules are essential proteins found on the surface of antigen-presenting cells (APCs) that provide a second signal necessary for T cell activation and function. This second signal is critical in the immune response, ensuring that T cells are fully activated only when there is a legitimate threat, such as an infection. The interaction between costimulatory molecules on APCs and their receptors on T cells helps to amplify the immune response and maintain tolerance to self-antigens, connecting the innate and adaptive immune systems.
Cross-presentation: Cross-presentation is a unique process where certain antigen-presenting cells (APCs) can present exogenous antigens on major histocompatibility complex (MHC) class I molecules to CD8+ T cells. This mechanism allows the immune system to recognize and respond to pathogens that do not infect APCs directly, bridging a crucial gap in the adaptive immune response. By facilitating the activation of CD8+ T cells, cross-presentation plays an essential role in the body's defense against viruses and tumors.
Cytokines: Cytokines are small signaling proteins that are crucial for cell communication in the immune system. They play an essential role in mediating and regulating immunity, inflammation, and hematopoiesis, linking innate and adaptive immune responses.
Defensins: Defensins are small cationic peptides that play a vital role in the innate immune system by exhibiting antimicrobial properties against bacteria, fungi, and viruses. They are produced by various cell types, including epithelial cells and leukocytes, and act as a first line of defense by disrupting microbial membranes and neutralizing pathogens. Defensins are essential for maintaining host defense mechanisms and contribute significantly to the overall response of the innate immune system.
Degranulation: Degranulation is the process by which immune cells, particularly mast cells and basophils, release their pre-stored granules containing various biologically active substances, including histamines and cytokines. This process plays a crucial role in the body's immediate response to pathogens and allergens, leading to inflammation and recruitment of other immune cells to the site of infection or injury. Degranulation is central to both innate immunity and allergic responses, highlighting its importance in various hypersensitivity reactions.
Dendritic Cells: Dendritic cells are a type of immune cell that play a crucial role in the body's immune response by capturing, processing, and presenting antigens to T cells. These cells serve as a bridge between the innate and adaptive immune systems, facilitating the activation of T cells and promoting the development of adaptive immunity.
Endocytosis: Endocytosis is a cellular process in which substances are brought into the cell by engulfing them within a membrane-bound vesicle. This mechanism is essential for innate immunity as it allows immune cells to internalize pathogens, debris, and other molecules, promoting effective responses against infections and maintaining homeostasis. Through various forms of endocytosis, cells can regulate their internal environment and communicate effectively with their surroundings.
Eosinophils: Eosinophils are a type of white blood cell that play a crucial role in the immune response, particularly in combating parasitic infections and participating in allergic reactions. These cells are characterized by their bilobed nucleus and distinctive granules that stain bright red with eosin dye, which is how they got their name. They contribute to innate immunity and can influence inflammation and tissue repair processes.
Granzymes: Granzymes are serine proteases that are released by cytotoxic T lymphocytes and natural killer (NK) cells to induce apoptosis, or programmed cell death, in target cells. These enzymes play a critical role in the immune response by effectively eliminating infected or cancerous cells, thereby maintaining the integrity of the organism. Their mechanism involves entering target cells through perforin-formed pores, leading to a cascade of events that ultimately trigger cell death.
Interferons: Interferons are a group of signaling proteins produced by host cells in response to viral infections and other pathogens. They play a critical role in the immune response by enhancing the antiviral defenses of neighboring cells, activating immune cells, and modulating the activity of the immune system, making them essential for both innate immunity and adaptive immune regulation.
Killer cell immunoglobulin-like receptors (KIRs): Killer cell immunoglobulin-like receptors (KIRs) are a family of receptors expressed on natural killer (NK) cells and some T cells, playing a critical role in regulating immune responses. They are primarily involved in distinguishing between healthy and infected or transformed cells, thereby influencing the cytotoxic activity of NK cells. KIRs interact with specific molecules on target cells, providing a balance between activating and inhibitory signals that determine whether an NK cell will attack a target.
Lysozyme: Lysozyme is an enzyme that breaks down the cell walls of certain bacteria, leading to their destruction. It plays a crucial role in the innate immune system as a natural antimicrobial agent, primarily found in secretions like saliva, tears, and mucus. By targeting peptidoglycan, a component of bacterial cell walls, lysozyme helps protect the body from infections.
Macrophages: Macrophages are large immune cells that play a crucial role in the body's defense mechanisms by engulfing and digesting cellular debris, pathogens, and foreign substances. They originate from monocytes in the blood and are pivotal in both the innate and adaptive immune responses, acting as key players in inflammation, antigen presentation, and tissue repair.
Mast Cells: Mast cells are a type of white blood cell that plays a crucial role in the immune system, particularly in allergic reactions and defense against parasites. They are found in various tissues, especially in the skin, lungs, and digestive tract, where they release histamine and other chemicals that contribute to inflammation and immune responses.
Memory-like properties: Memory-like properties refer to the ability of certain components of the immune system to exhibit a form of 'memory' without the classic characteristics associated with adaptive immunity. This means that some innate immune cells can respond more effectively upon re-exposure to pathogens, even though they don't possess the antigen-specific memory seen in B and T cells. This phenomenon is particularly relevant in understanding how the innate immune system can adapt and enhance its response over time, contributing to overall immune protection.
Myeloperoxidase: Myeloperoxidase (MPO) is an enzyme found primarily in neutrophils and monocytes, crucial for the innate immune response. This enzyme plays a significant role in generating reactive oxygen species during the respiratory burst, which helps kill pathogens. By converting hydrogen peroxide into hypochlorous acid, myeloperoxidase acts as a potent antimicrobial agent that contributes to the body's first line of defense against infections.
Natural killer cells: Natural killer (NK) cells are a type of lymphocyte in the innate immune system that play a crucial role in the body’s defense against tumors and virally infected cells. They act quickly to identify and eliminate these threats without the need for prior sensitization, which makes them essential for immediate immune responses. Their ability to recognize stressed cells in the absence of antibodies or major histocompatibility complex (MHC) molecules highlights their unique function in immune surveillance and responses to cellular abnormalities.
Neutrophil Extracellular Traps (NETs): Neutrophil extracellular traps (NETs) are web-like structures composed of DNA and antimicrobial proteins that are released by activated neutrophils in response to infection. These structures trap and kill pathogens, playing a critical role in the innate immune response by preventing the spread of microorganisms and facilitating their clearance from the body.
Neutrophils: Neutrophils are a type of white blood cell that plays a vital role in the innate immune response, acting as the first line of defense against invading pathogens. They are essential for responding quickly to infections, especially bacterial and fungal, and are characterized by their ability to migrate to sites of inflammation and infection through the bloodstream and tissues.
Pathogen-associated molecular patterns (PAMPs): Pathogen-associated molecular patterns (PAMPs) are conserved molecular structures found on pathogens that are recognized by the innate immune system. They serve as signals that alert immune cells to the presence of infections, helping the body mount an immediate defense against various types of pathogens, including bacteria, viruses, and fungi. PAMPs play a crucial role in the activation of innate immune responses and are essential for the recognition of foreign invaders.
Pattern Recognition Receptors (PRRs): Pattern Recognition Receptors (PRRs) are a group of proteins expressed by cells of the immune system that recognize specific patterns associated with pathogens or damage. These receptors are crucial for the innate immune response, enabling cells to detect infections and initiate appropriate immune reactions. PRRs play a pivotal role in bridging innate and adaptive immunity, influencing inflammation and the activation of various immune cells.
Perforin: Perforin is a protein produced by cytotoxic T cells and natural killer (NK) cells that plays a vital role in the immune response by forming pores in the membranes of target cells. This pore-forming ability allows for the entry of granzymes, which are enzymes that induce apoptosis, or programmed cell death, in infected or cancerous cells. The action of perforin is essential for eliminating threats to the body, highlighting its importance in innate immunity.
Phagocytosis: Phagocytosis is the process by which certain cells, known as phagocytes, engulf and digest foreign particles, pathogens, or cellular debris. This crucial mechanism is a key component of the innate immune response, aiding in the clearance of infections and the maintenance of tissue homeostasis. Phagocytosis connects to various aspects of immune function, including the role of immune cells in recognizing threats, the activation of signaling pathways, and interactions with other immune components, such as the complement system.
Reactive Oxygen Species (ROS): Reactive oxygen species (ROS) are highly reactive molecules derived from oxygen that play a significant role in cellular signaling and homeostasis. They can be both beneficial and harmful, as they are involved in processes like immune defense and apoptosis, but excessive levels can lead to oxidative stress, damaging cellular components and contributing to various diseases, including cancer and neurodegeneration.
Tolerance Induction: Tolerance induction refers to the process by which the immune system learns to recognize and accept specific antigens without mounting an immune response. This mechanism is crucial for maintaining self-tolerance and preventing autoimmune diseases, as well as allowing the body to tolerate harmless antigens such as food proteins or commensal microbiota. Through various cellular interactions, tolerance induction helps in establishing a balance between immunity and tolerance, particularly involving key players of the immune system.