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.
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Neutrophils in the activation and regulation of innate and adaptive immunity. 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. The engulfed pathogen is kept in a vacuole called a phagosome, which then binds to the lysosomes inside the cell.
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