Function Of White Blood Cells In The Immune System

Function Of White Blood Cells In The Immune System – Much information about the various organ systems of vertebrates is not within the AP

. However, the immune system was chosen for in-depth exploration because all organisms, including humans, must maintain dynamic homeostasis to survive in changing environments. Even the simplest multicellular eukaryotes, such as fungi and Cnidarians, have evolved cells that specialize in immune defenses to protect against disruption of homeostasis. News headlines warn us of disease outbreaks, including Ebola, measles, influenza, and insect-borne viruses such as West Nile and chikungunya, that spread rapidly through populations, often with devastating consequences. We are also hearing about the emergence of new infections, especially those caused by bacteria that have developed resistance to antibiotics.

Function Of White Blood Cells In The Immune System

Function Of White Blood Cells In The Immune System

Immune systems in animals range from the loose cluster of phagocytic cells in fungi to the complex interactions of molecules, cells, tissues, and organs that provide immunity to mammals. Components of the immune system are constantly searching the body for signs of disease-causing microorganisms called pathogens. Immune factors are mobilized, identify the nature of the pathogen, strengthen the appropriate cells and molecules to fight the infection, and after the infection is cleared, shut down the immune response to prevent unnecessary damage to the host cell. Thanks to its programmable memory system, the immune system can remember pathogens and initiate a faster response after re-exposure. The immune response can be either innate or adaptive. The adaptive immune response stores information about past infections and builds pathogen-specific defenses. The innate immune response is always present and defends against all pathogens.

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Despite the barriers of skin, tears and mucus, pathogens can still enter the body. The innate immune system responds with inflammation, engulfment of pathogens, and secretion of immune factors and proteins. Several cell types are involved in the innate immune system, including mast cells that release histamines (causing unpleasant symptoms associated with allergies and the common cold), macrophages that consume pathogens and cancer cells, natural killer (NK) cells that destroy tumor cells, and viruses . – infected cells, several types of white blood cells and even protective proteins such as complement and interferon. However, we know from experience that these barriers can fail. Fortunately, adaptive immune responses provide another, more specific line of defense.

The information presented and examples highlighted in the section support the concepts outlined in the Big Idea 2 AP

Biological systems use free energy and molecular building blocks to grow, reproduce, and maintain dynamic homeostasis.

1.1 The student can create representations and models of natural or artificial phenomena and systems in the field.

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1.2 The student can describe representations and models of natural or artificial phenomena and systems in the field.

The immune system includes both innate and adaptive immune responses. Innate immunity occurs naturally due to genetic factors or physiology; it is not induced by infection or vaccination but acts to reduce the workload for the adaptive immune response. Both the innate and adaptive levels of the immune response involve secreted proteins, receptor-mediated signaling, and complex cell-to-cell communication. The innate immune system developed early in animal evolution, roughly a billion years ago, as a basic response to infection. Innate immunity has a limited number of specific targets: any pathogenic threat triggers a consistent sequence of events that can identify the type of pathogen and either clear the infection independently or mobilize a highly specialized adaptive immune response. For example, tears and mucus secretions contain microbicidal factors.

Before any immune factors are triggered, the skin acts as a continuous, impermeable barrier to potentially infectious pathogens. Pathogens are killed or inactivated on the skin by desiccation (drying) and skin acidity. In addition, the beneficial microorganisms that coexist on the skin compete with invading pathogens and prevent infection. Areas of the body that are not protected by the skin (such as the eyes and mucous membranes) have alternative defenses, such as tears and mucus secretions that trap and wash away pathogens, and cilia in the nasal passages and airways that push pathogen-laden mucus out of the body . There are other defenses throughout the body, such as the low pH of the stomach (which inhibits the growth of pathogens), blood proteins that bind to and disrupt bacterial cell membranes, and the process of urination (which flushes pathogens from the urinary tract).

Function Of White Blood Cells In The Immune System

Despite these barriers, pathogens can enter the body through skin abrasions or punctures, or by collecting on mucosal surfaces in large numbers that overcome mucus or cilia. Some pathogens have evolved specific mechanisms that allow them to overcome physical and chemical barriers. When pathogens enter the body, the innate immune system responds with inflammation, engulfment of the pathogen, and secretion of immune factors and proteins.

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Infection can be intracellular or extracellular depending on the pathogen. All viruses infect cells and replicate within those cells (intracellularly), while bacteria and other parasites can replicate intracellularly or extracellularly, depending on the species. The innate immune system must respond accordingly: by identifying the extracellular pathogen and/or by identifying host cells that have already been infected. When a pathogen enters the body, cells in the blood and lymph detect specific pathogen-associated molecular patterns (PAMPs) on the surface of the pathogen. PAMPs are carbohydrates, polypeptides, and nucleic acids

Which are expressed by viruses, bacteria and parasites, but which differ from molecules on host cells. The immune system has specific cells, described in Figure 33.2 and shown in Figure 33.3, with receptors that recognize these PAMPs. A macrophage is a large phagocytic cell that engulfs foreign particles and pathogens. Macrophages recognize PAMPs through complement pattern recognition receptors (PRRs). PRRs are molecules on macrophages and dendritic cells that are in contact with the external environment. A monocyte is a type of white blood cell that circulates in the blood and lymph and differentiates into macrophages after moving into infected tissue. Dendritic cells bind molecular signatures of pathogens and promote pathogen engulfment and destruction. Toll-like receptors (TLRs) are a type of PRR that recognize molecules shared by pathogens but distinguishable from host molecules. TLRs are present in both invertebrates and vertebrates and appear to be one of the oldest components of the immune system. TLRs have also been identified in the mammalian nervous system.

Figure 33.2 The characteristics and location of cells involved in the innate immune system are described. (credit: adaptation of work by NIH)

Figure 33.3 Blood cells include (1) monocytes, (2) lymphocytes, (3) neutrophils, (4) red blood cells, and (5) platelets. Note the very similar leukocyte morphology (1, 2, 3). (credits: adaptation of work by Bruce Wetzel, Harry Schaefer, NCI; scale data by Matt Russell)

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Binding of PRRs to PAMPs triggers the release of cytokines that signal that a pathogen is present and must be destroyed along with the infected cells. A cytokine is a chemical messenger that regulates cell differentiation (form and function), proliferation (production), and gene expression to influence immune responses. In humans, there are at least 40 types of cytokines, which differ in the type of cell that produces them, the type of cell that responds to them, and the changes they produce. One type of cytokine, interferon, is shown in Figure 33.4.

One subclass of cytokines is interleukin (IL), so named because it mediates interactions between leukocytes (white blood cells). Interleukins are involved in bridging innate and adaptive immune responses. In addition to being released from cells upon PAMP recognition, cytokines are released by infected cells that bind to nearby uninfected cells and induce these cells to release cytokines, resulting in a cytokine burst.

A second class of early-acting cytokines are interferons, which are released by infected cells as a warning to surrounding uninfected cells. One of the functions of interferon is to inhibit virus replication. They also have other important functions such as tumor monitoring. Interferons work by signaling neighboring uninfected cells to destroy RNA and reduce protein synthesis, signaling neighboring infected cells to undergo apoptosis (programmed cell death), and activating immune cells.

Function Of White Blood Cells In The Immune System

In response to interferons, uninfected cells change their gene expression, increasing the cells’ resistance to infection. One of the effects of interferon-induced gene expression is severely reduced cellular protein synthesis. Virus-infected cells produce more virus by synthesizing large amounts of viral proteins. Thus, by reducing protein synthesis, the cell becomes resistant to viral infection.

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Figure 33.4 Interferons are cytokines that are released by a virus-infected cell. The response of neighboring cells to interferon helps stop the infection.

The first cytokines produced are pro-inflammatory; that is, they promote inflammation, localized redness, swelling, heat, and pain that result from the movement of leukocytes and fluid through increasingly permeable capillaries to the site of infection. The population of leukocytes that reaches the site of infection depends on the nature of the infectious pathogen. Both macrophages and dendritic cells engulf pathogens and cellular debris through phagocytosis. A neutrophil is also a phagocytic leukocyte that engulfs and digests pathogens. Neutrophils, shown in Figure 33.3, are the most abundant leukocytes of the immune system. Neutrophils have a nucleus with two to five lobes and contain organelles called lysosomes that digest ingested pathogens. An eosinophil is a leukocyte that cooperates with others

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