Dendritic cell : Dendritic cell characterized by membranous projections that resemble spines. Dendritic cells are specialized antigen-presenting cells that have long outgrowths called dendrites, which help to engulf microbes and other invaders. Dendritic cells are present in the tissues that are in contact with the external environment, mainly the skin, the inner lining of the nose, the lungs, the stomach, and the intestines. Once activated, they mature and migrate to the lymphoid tissues, where they present antigens to T and B cells to initiate the adaptive immune response.
This involves deriving T and B cells that are specific towards a single antigen from naive lymphocytes. Neutrophils are a type of PMN granulocyte normally found in the bloodstream. They are the most abundant type of phagocyte and the first responder during inflammation.
Once they have received the appropriate chemokine signals, neutrophils leave the bloodstream and reach the site of an infection through adhering to the vascular endothelium to squeeze into the tissues. There, they rapidly engulf invaders coated with antibodies, damaged cells, or cellular debris.
They also degranulate to release perforin, granzyme, proteases, and other chemicals to cause cytotoxic damage to pathogens and occasionally normal bodily tissues as well. Neutrophils die after phagocytosis, becoming pus that is later cleaned up by macrophages. Extravasion of Neutrophils : Neutrophils move through the blood to the site of infection by rolling onto the vascular endothelium and adhering to it to slip through small gaps into the tissues during an inflammatory response.
Mast cells are PMN granulocytes with toll-like receptors that tend to trigger inflammatory responses. Mast cells can consume, kill, and process their antigens.
In addition to these functions, mast cells produce cytokines kept in their granules, such as histamine, that induce an inflammatory response when a pathogen is detected. Leukocyte Differentiation : Phagocytes derive from stem cells in the bone marrow. Monocytes differentiate into dendritic cells and macrophages, while mast cells and neutrophils are in a separate group of PMN granuolcytes as well.
Learning Objectives Describe the types of phagocytes and their roles in initiating an immune response. Key Points Many white blood cells and other cells in the body use phagocytosis to engulf and kill cells.
Phagocytosis occurs over several steps, which include binding to an opsonized pathogen with a receptor and killing it using an oxidative burst. Monocytes are phagocytes that can differentiate into macrophages and dendritic cells based on conditions within the body. Macrophages are the cleanup crew for the innate immune system.
They remove debris, pathogens, and dead neutrophils after an inflammatory response. Neutrophils are polymorphonuclear PMN granulocytes that are the first responders to an inflammatory response. They kill pathogens through phagocytosis and degranulation, but die as a result.
Mast cells are circulation PMN granulocytes that trigger an immune response by releasing an inflammatory mediator when they detect an antigen with their toll-like receptors. Key Terms oxidative burst : A chemical reaction that occurs in phagocytes in which an engulfed pathogen is destroyed by exposure to oxidative stress from reactive oxygen species.
PMN granulocyte : A type of phagocyte that contains PMN granules, most notably neutrophils and mast cells, but also basophiles and eosinophils. The Mechanism of Phagocytosis Phagocytosis is the process by which a phagocyte engulfs a pathogen or debris. The Fc receptor is typically the receptor of use, which binds to antibodies that have opsonized marked a pathogen or toxin.
The cytoplasm surrounds and engulfs the bound pathogen through endocytosis. In the tissues, they are activated by cytokines and arrive at the battle scene ready to kill. Neutrophils : Neutrophils move through the blood to the site of infection.
These chemical signals may include proteins from invading bacteria, clotting system peptides, complement products, and cytokines that have been given off by macrophages located in the tissue near the infection site. Another group of chemical attractants are cytokines that recruit neutrophils and monocytes from the blood. To reach the site of infection, phagocytes leave the bloodstream and enter the affected tissues. Signals from the infection cause the endothelial cells that line the blood vessels to make a protein called selectin, which neutrophils stick to when they pass by.
Other signals called vasodilators loosen the junctions connecting endothelial cells, allowing the phagocytes to pass through the wall. Neutrophils travel across epithelial cell-lined organs to sites of infection, and although this is an important component of fighting infection, the migration itself can result in disease-like symptoms.
During an infection, millions of neutrophils are recruited from the blood, but they die after a few days. B and T cells, parts of the adaptive immune response, contain receptors that can identify antigens derived from pathogens.
The adaptive, or acquired, immune response to an initial infection takes days or even weeks to become established, much longer than the innate response. However, adaptive immunity is more specific to an invading pathogen and can fight back much more quickly than the innate response if it has seen the pathogen before. Adaptive immunity occurs after exposure to an antigen either from a pathogen or a vaccination. An antigen is a molecule that binds to a specific antibody, often stimulating a response in the immune system as a result.
The adaptive immune response activates when the innate immune response insufficiently controls an infection. In fact, without information from the innate immune system, the adaptive response could not be mobilized.
There are two types of adaptive responses: the cell-mediated immune response, which is controlled by activated T cells, and the humoral immune response, which is controlled by activated B cells and antibodies. Upon infection, activated T and B cells that have surface binding sites with specificity to the molecules on the pathogen greatly increase in number and attack the invading pathogen.
Their attack can kill pathogens directly or they can secrete antibodies that enhance the phagocytosis of pathogens and disrupt the infection. Adaptive immunity also involves a memory, which gives the host long-term protection from reinfection by the same type of pathogen; upon re-exposure, this host memory will facilitate a rapid and powerful response. Lymphocytes, which are white blood cells, are formed with other blood cells in the red bone marrow found in many flat bones, such as the shoulder or pelvic bones.
The two types of lymphocytes of the adaptive immune response are B and T cells. Whether an immature lymphocyte becomes a B cell or T cell depends on where in the body it matures. T and B cells are indistinguishable by light microscopy, but can be differentiated experimentally by probing their surface receptors. The maturation of a B or T cell involves becoming immunocompetent, meaning that it can recognize and bind to a specific molecule or antigen.
This recognition, which is central to the functioning of the adaptive immune response, results from the presence of highly specific receptors on the surfaces of B and T cells. On B cells, these receptors contain antibodies, which are responsible for antigen binding. An antibody is specific for one particular antigen; typically, it will not bind to anything else.
Upon antigen binding to a B cell receptor, a signal is sent into the B cell to turn on an immune response. B cell receptors : B cell receptors are embedded in the membranes of B cells and bind a variety of antigens through their variable regions, or antibodies. The signal transduction region transfers the signal into the cell. Meanwhile, T cell receptors are responsible for the recognition of pathogenic antigens by T cells.
Unlike B cells, T cells do not directly recognize antigens. Instead, they recognize antigens presented on major histocompatibility complexes MHCs that cells use to display which proteins are inside of them. If a cell is infected, it will present antigenic portions of the infecting pathogen on its MHC for recognition by T cells, which will then mount an appropriate immune response.
Unlike antibodies, which can typically bind one and only one antigen, T cell receptors have more flexibility in their capacity to recognize antigens presented by MHCs. It is the specific pathogen recognition via binding antigens of B and T cells that allows the adaptive immune response to adapt.
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