Thus, even though the initial cell is sacrificed, the surrounding cells are protected. Other early induced proteins specific for bacterial cell wall components are mannose-binding protein and C-reactive protein, made in the liver, which bind specifically to polysaccharide components of the bacterial cell wall. Phagocytes such as macrophages have receptors for these proteins, and they are thus able to recognize them as they are bound to the bacteria.
This brings the phagocyte and bacterium into close proximity and enhances the phagocytosis of the bacterium by the process known as opsonization. Opsonization is the tagging of a pathogen for phagocytosis by the binding of an antibody or an antimicrobial protein.
The complement system is a series of proteins constitutively found in the blood plasma. As such, these proteins are not considered part of the early induced immune response , even though they share features with some of the antibacterial proteins of this class.
Additionally, complement functions in the adaptive immune response as well, in what is called the classical pathway. The complement system consists of several proteins that enzymatically alter and fragment later proteins in a series, which is why it is termed cascade.
Once activated, the series of reactions is irreversible, and releases fragments that have the following actions:. Figure 2 shows the classical pathway, which requires antibodies of the adaptive immune response.
The alternate pathway does not require an antibody to become activated. Figure 2. The classical pathway, used during adaptive immune responses, occurs when C1 reacts with antibodies that have bound an antigen.
The splitting of the C3 protein is the common step to both pathways. In the alternate pathway, C3 is activated spontaneously and, after reacting with the molecules factor P, factor B, and factor D, splits apart. The larger fragment, C3b, binds to the surface of the pathogen and C3a, the smaller fragment, diffuses outward from the site of activation and attracts phagocytes to the site of infection. Surface-bound C3b then activates the rest of the cascade, with the last five proteins, C5—C9, forming the membrane-attack complex MAC.
The MAC can kill certain pathogens by disrupting their osmotic balance. The MAC is especially effective against a broad range of bacteria. The classical pathway is similar, except the early stages of activation require the presence of antibody bound to antigen, and thus is dependent on the adaptive immune response.
The earlier fragments of the cascade also have important functions. Phagocytic cells such as macrophages and neutrophils are attracted to an infection site by chemotactic attraction to smaller complement fragments. Additionally, once they arrive, their receptors for surface-bound C3b opsonize the pathogen for phagocytosis and destruction.
The hallmark of the innate immune response is inflammation. Inflammation is something everyone has experienced. It is important to note that inflammation does not have to be initiated by an infection, but can also be caused by tissue injuries. The release of damaged cellular contents into the site of injury is enough to stimulate the response, even in the absence of breaks in physical barriers that would allow pathogens to enter by hitting your thumb with a hammer, for example.
The inflammatory reaction brings in phagocytic cells to the damaged area to clear cellular debris and to set the stage for wound repair Figure 3. This reaction also brings in the cells of the innate immune system, allowing them to get rid of the sources of a possible infection. Inflammation is part of a very basic form of immune response.
The process not only brings fluid and cells into the site to destroy the pathogen and remove it and debris from the site, but also helps to isolate the site, limiting the spread of the pathogen. Acute inflammation is a short-term inflammatory response to an insult to the body.
If the cause of the inflammation is not resolved, however, it can lead to chronic inflammation, which is associated with major tissue destruction and fibrosis. Chronic inflammation is ongoing inflammation. It can be caused by foreign bodies, persistent pathogens, and autoimmune diseases such as rheumatoid arthritis.
Overall, inflammation is valuable for many reasons. Not only are the pathogens killed and debris removed, but the increase in vascular permeability encourages the entry of clotting factors, the first step towards wound repair. Inflammation also facilitates the transport of antigen to lymph nodes by dendritic cells for the development of the adaptive immune response.
Innate immune responses are critical to the early control of infections. Innate responses occur rapidly, but with less specificity and effectiveness than the adaptive immune response. Innate responses can be caused by a variety of cells, mediators, and antibacterial proteins such as complement.
Within the first few days of an infection, another series of antibacterial proteins are induced, each with activities against certain bacteria, including opsonization of certain species.
Additionally, interferons are induced that protect cells from viruses in their vicinity. Finally, the innate immune response does not stop when the adaptive immune response is developed. In fact, both can cooperate and one can influence the other in their responses against pathogens. Answer the question s below to see how well you understand the topics covered in the previous section. Skip to main content. Module 5: The Lymphatic and Immune System.
Search for:. Barrier Defenses and the Innate Immune Response Learning Objectives By the end of this section, you will be able to: Describe the barrier defenses of the body Show how the innate immune response is important and how it helps guide and prepare the body for adaptive immune responses Describe various soluble factors that are part of the innate immune response Explain the steps of inflammation and how they lead to destruction of a pathogen Discuss early induced immune responses and their level of effectiveness.
Practice Question Visit this website to learn about phagocyte chemotaxis. Figure 3. The Inflammatory Process. Critical Thinking Questions Describe the process of inflammation in an area that has been traumatized, but not infected. Describe two early induced responses and what pathogens they affect.
Show Answers Interferons are produced in virally infected cells and cause them to secrete signals for surrounding cells to make antiviral proteins. C-reactive protein is induced to be made by the liver and will opsonize certain species of bacteria.
The cell debris and damaged cells induce macrophages to begin to clean them up. Macrophages release cytokines that attract neutrophils, followed by more macrophages. A second, thicker layer, called the dermis, contains hair follicles, sweat glands, nerves, and blood vessels. A layer of fatty tissue called the hypodermis lies beneath the dermis and contains blood and lymph vessels Figure 2. The topmost layer of skin, the epidermis , consists of cells that are packed with keratin.
These dead cells remain as a tightly connected, dense layer of protein-filled cell husks on the surface of the skin. In addition, the dead cells of the epidermis are frequently shed, along with any microbes that may be clinging to them.
Shed skin cells are continually replaced with new cells from below, providing a new barrier that will soon be shed in the same way.
Infections can occur when the skin barrier is compromised or broken. A wound can serve as a point of entry for opportunistic pathogens, which can infect the skin tissue surrounding the wound and possibly spread to deeper tissues. Mike, a gardener from southern California, recently noticed a small red bump on his left forearm. Initially, he did not think much of it, but soon it grew larger and then ulcerated opened up , becoming a painful lesion that extended across a large part of his forearm Figure 3.
He went to an urgent care facility, where a physician asked about his occupation. Figure 3. Under most conditions, fungi cannot produce skin infections in healthy individuals. Fungi grow filaments known as hyphae, which are not particularly invasive and can be easily kept at bay by the physical barriers of the skin and mucous membranes. Once it breaches the skin barrier, S.
Compounding matters, other pathogens may enter the infected tissue, causing secondary bacterial infections. His lesions eventually healed, and Mike returned to work with a new appreciation for gloves and protective clothing. The mucous membrane s lining the nose, mouth, lungs, and urinary and digestive tracts provide another nonspecific barrier against potential pathogens. Mucous membranes consist of a layer of epithelial cells bound by tight junctions.
The epithelial cells secrete a moist, sticky substance called mucus , which covers and protects the more fragile cell layers beneath it and traps debris and particulate matter, including microbes. Mucus secretions also contain antimicrobial peptides. Figure 4. This scanning electron micrograph shows ciliated and nonciliated epithelial cells from the human trachea.
The mucociliary escalator pushes mucus away from the lungs, along with any debris or microorganisms that may be trapped in the sticky mucus, and the mucus moves up to the esophagus where it can be removed by swallowing. In many regions of the body, mechanical actions serve to flush mucus along with trapped or dead microbes out of the body or away from potential sites of infection.
For example, in the respiratory system, inhalation can bring microbes, dust, mold spores, and other small airborne debris into the body. This debris becomes trapped in the mucus lining the respiratory tract, a layer known as the mucociliary blanket.
The epithelial cells lining the upper parts of the respiratory tract are called ciliated epithelial cells because they have hair-like appendages known as cilia. Movement of the cilia propels debris-laden mucus out and away from the lungs. The expelled mucus is then swallowed and destroyed in the stomach, or coughed up, or sneezed out Figure 4.
This system of removal is often called the mucociliary escalator. The mucociliary escalator is such an effective barrier to microbes that the lungs, the lowermost and most sensitive portion of the respiratory tract, were long considered to be a sterile environment in healthy individuals.
Only recently has research suggested that healthy lungs may have a small normal microbiota. Disruption of the mucociliary escalator by the damaging effects of smoking or diseases such as cystic fibrosis can lead to increased colonization of bacteria in the lower respiratory tract and frequent infections, which highlights the importance of this physical barrier to host defenses.
Like the respiratory tract, the digestive tract is a portal of entry through which microbes enter the body, and the mucous membranes lining the digestive tract provide a nonspecific physical barrier against ingested microbes. The intestinal tract is lined with epithelial cells, interspersed with mucus-secreting goblet cells Figure 5.
This mucus mixes with material received from the stomach, trapping foodborne microbes and debris. The mechanical action of peristalsis , a series of muscular contractions in the digestive tract, moves the sloughed mucus and other material through the intestines, rectum, and anus, excreting the material in feces.
Figure 5. Goblet cells produce and secrete mucus. The epithelial cells lining the urogenital tract, blood vessels, lymphatic vessels, and certain other tissues are known as endothelia. These tightly packed cells provide a particularly effective frontline barrier against invaders. The endothelia of the blood-brain barrier , for example, protect the central nervous system CNS , which consists of the brain and the spinal cord. The CNS is one of the most sensitive and important areas of the body, as microbial infection of the CNS can quickly lead to serious and often fatal inflammation.
The cell junctions in the blood vessels traveling through the CNS are some of the tightest and toughest in the body, preventing any transient microbes in the bloodstream from entering the CNS.
This keeps the cerebrospinal fluid that surrounds and bathes the brain and spinal cord sterile under normal conditions. In addition to physical barriers that keep microbes out, the body has a number of mechanical defenses that physically remove pathogens from the body, preventing them from taking up residence. We have already discussed several examples of mechanical defenses, including the shedding of skin cells, the expulsion of mucus via the mucociliary escalator, and the excretion of feces through intestinal peristalsis.
Other important examples of mechanical defenses include the flushing action of urine and tears, which both serve to carry microbes away from the body. The flushing action of urine is largely responsible for the normally sterile environment of the urinary tract, which includes the kidneys, ureters, and urinary bladder.
Urine passing out of the body washes out transient microorganisms, preventing them from taking up residence. The eyes also have physical barriers and mechanical mechanisms for preventing infections. The eyelashes and eyelids prevent dust and airborne microorganisms from reaching the surface of the eye. Any microbes or debris that make it past these physical barriers may be flushed out by the mechanical action of blinking, which bathes the eye in tears, washing debris away Figure 6.
Figure 6. Tears flush microbes away from the surface of the eye. Urine washes microbes out of the urinary tract as it passes through; as a result, the urinary system is normally sterile. In various regions of the body, resident microbiota serve as an important first-line defense against invading pathogens.
Through their occupation of cellular binding sites and competition for available nutrients, the resident microbiota prevent the critical early steps of pathogen attachment and proliferation required for the establishment of an infection. For example, in the vagina , members of the resident microbiota compete with opportunistic pathogens like the yeast Candida.
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