up:: [[Immune System]] x:: [[Lymphatic System]] tags:: #on/physiology #on/anatomy #on/immunity # Innate Immune System The innate immune system serves as a primary defense mechanism against pathogens in vertebrates. It operates through various strategies such as *recruiting immune cells to infection sites, activating the complement cascade, removing foreign substances, acting as a barrier, and initiating the adaptive immune response* through antigen presentation. Anatomical barriers, including physical, chemical, and biological barriers, play a crucial role in preventing infection in the skin, gastrointestinal tract, respiratory airways, eyes, and other organs. These barriers involve mechanisms like desquamation, mucus, peristalsis, gastric acid, and tears. ### Inflammation Inflammation is a key response of the innate immune system to infection or irritation. It involves the release of chemical factors by injured cells, leading to *increased blood circulation, local vasodilation, swelling, mucus production, and pain*. The [[Complement System]] complements antibody actions by triggering inflammatory responses, tagging pathogens for destruction, forming holes in pathogen membranes, and removing neutralized antigen-antibody complexes. ### White Blood Cells (see [[Leukocyte]]) White blood cells, including mast cells, phagocytes (macrophages, neutrophils, dendritic cells), basophils, eosinophils, and natural killer cells, are vital components of the innate immune system. These cells function in detecting and eliminating pathogens through mechanisms such as phagocytosis, respiratory burst, and the release of inflammatory mediators. *Additionally, γδ T cells and other cell subsets contribute to the immune response, blurring the line between innate and adaptive immunity*. The innate immune system displays specificity for different pathogens, employing various mechanisms to combat intracellular and extracellular threats. However, pathogens have evolved strategies to evade immune detection and response, such as *intracellular replication, biofilm formation, and inhibiting phagocytosis*. Understanding these evasion tactics is crucial for developing effective treatments against infections. **Key Points:** - Anatomical barriers and inflammation are essential components of innate immunity, aiding in pathogen detection and elimination. - The innate immune system acts as a primary defense mechanism against pathogens in vertebrates and other organisms. - White blood cells, including phagocytes, mast cells, and natural killer cells, play crucial roles in innate immunity. - Pathogens have evolved evasion tactics to evade immune detection and response, highlighting the importance of understanding these mechanisms for developing treatments against infections. ### Immune Sensing The innate immune system utilizes pattern recognition receptors to identify pathogens and damaged cells through PAMPs and DAMPs. *Toll-like receptors (TLRs) on cell surfaces or within endosomes detect extracellular or endosomal PAMPs, triggering an immune response*. Additionally, cytosolic receptors like NOD-like and RIG-like receptors, along with cytosolic DNA sensors, play a crucial role in recognizing internal threats, initiating defense mechanisms against infections. - Pattern recognition receptors detect pathogen-associated and damage-associated molecular patterns. - Toll-like receptors (TLRs) initiate immune responses by recognizing PAMPs. - Cytosolic receptors such as NOD-like and RIG-like identify internal threats. ### Innate Immune Cells Innate immune cells, including various [[Leukocytes]] like phagocytes, dendritic cells, and natural killer (NK) cells, form the first line of defense against pathogens. Phagocytes engulf and destroy pathogens, while dendritic cells act as a bridge to the [[Adaptive Immune System]]. NK cells target and kill compromised host cells, contributing to the body's defense mechanism against infections and tumors. - Leukocytes such as phagocytes, dendritic cells, and NK cells are crucial for pathogen elimination. - Phagocytes engulf pathogens; dendritic cells present antigens to T cells. - NK cells destroy cells with low levels of MHC I, preventing viral infections and tumor growth. ### Inflammation [[Inflammation]] is an essential response to infection, characterized by symptoms such as redness, swelling, and pain. This response is mediated by [[Cytokines]] and [[Eicosanoids]] released by injured or infected cells, facilitating the recruitment of immune cells to the site of infection and promoting tissue repair. Inflammasomes, which form in response to PAMPs and DAMPs, play a significant role in generating inflammatory cytokines. - Inflammation is triggered by cytokines and eicosanoids from damaged cells. - Symptoms include redness, swelling, heat, and pain due to increased blood flow. - Inflammasomes generate inflammatory cytokines, contributing to the immune response. ### Humoral Defenses The complement system, part of the humoral defenses, plays a pivotal role in the innate immune response by attacking foreign cell surfaces through a biochemical cascade. It is *activated by antibodies bound to pathogens or directly by pathogen surfaces*, leading to the rapid elimination of invaders. This cascade amplifies the initial signal, attracting immune cells, and marking pathogens for destruction. - The complement system attacks foreign cells and is activated by antibodies or pathogen surfaces. - It amplifies the immune response through a proteolytic cascade. - Results in the attraction of immune cells, increased vascular permeability, and pathogen destruction.