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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.

Inflammatory response

The hallmark of the innate immune response is inflammation    . Inflammation is something everyone has experienced. Stub a toe, cut a finger, or do any activity that causes tissue damage and inflammation will result, with its four characteristics: heat, redness, pain, and swelling (“loss of function” is sometimes mentioned as a fifth characteristic). 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 ( [link] ).

The top panel of this figure shows the mast cells detecting an injury and initiating an inflammatory response. The bottom panel shows the increase in blood flow in response to histamine.

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.

There are four important parts to the inflammatory response:

  • Tissue Injury. The released contents of injured cells stimulate the release of mast cell    granules and their potent inflammatory mediators such as histamine, leukotrienes, and prostaglandins. Histamine increases the diameter of local blood vessels (vasodilation), causing an increase in blood flow. Histamine also increases the permeability of local capillaries, causing plasma to leak out and form interstitial fluid. This causes the swelling associated with inflammation.
    Additionally, injured cells, phagocytes, and basophils are sources of inflammatory mediators, including prostaglandins and leukotrienes. Leukotrienes attract neutrophils from the blood by chemotaxis and increase vascular permeability. Prostaglandins cause vasodilation by relaxing vascular smooth muscle and are a major cause of the pain associated with inflammation. Nonsteroidal anti-inflammatory drugs such as aspirin and ibuprofen relieve pain by inhibiting prostaglandin production.
  • Vasodilation. Many inflammatory mediators such as histamine are vasodilators that increase the diameters of local capillaries. This causes increased blood flow and is responsible for the heat and redness of inflamed tissue. It allows greater access of the blood to the site of inflammation.
  • Increased Vascular Permeability. At the same time, inflammatory mediators increase the permeability of the local vasculature, causing leakage of fluid into the interstitial space, resulting in the swelling, or edema, associated with inflammation.
  • Recruitment of Phagocytes. Leukotrienes are particularly good at attracting neutrophils from the blood to the site of infection by chemotaxis. Following an early neutrophil infiltrate stimulated by macrophage cytokines, more macrophages are recruited to clean up the debris left over at the site. When local infections are severe, neutrophils are attracted to the sites of infections in large numbers, and as they phagocytose the pathogens and subsequently die, their accumulated cellular remains are visible as pus at the infection site.

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.

Chapter review

Innate immune responses are critical to the early control of infections. Whereas barrier defenses are the body’s first line of physical defense against pathogens, innate immune responses are the first line of physiological defense. 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.

Visit this website to learn about phagocyte chemotaxis. Phagocyte chemotaxis is the movement of phagocytes according to the secretion of chemical messengers in the form of interleukins and other chemokines. By what means does a phagocyte destroy a bacterium that it has ingested?

The bacterium is digested by the phagocyte’s digestive enzymes (contained in its lysosomes).

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Questions & Answers

hello what's the topic
Parongu Reply
hello what's the top
Parongu
hello what's the top
Parongu
musah was admitted to your ward yesterday with traumatic amputation of the right thump . he complained this morning that he has not passed enough urine. Questions. 1) will you consider his complain to be pathological or physiological?. 2) in less than three sentences, justify your opinion. 3) Apply your understanding of the renin- angiotensin-aldosterone system to explain the factors and mechanisms accounting for his complains.
Ellen Reply
the mineral salt which break up a large portion of bone tissue is
Aurora Reply
calcium
Calcium
Sawda
please I need help. musah was admitted to your ward yesterday with traumatic amputation of the right thump. he complained this morning that he has not passed enough urine. Questions. 1). will you consider his complain to be pathological or physiological?.
Ellen
2). In less than three sentences, justify your opinion.
Ellen
Physiological
Ruth
cholesterol and triglycerides in simplest terms are fats (lipid) found in blood
Roy Reply
calcium is important for bone density and development. it also helps for safe / normal blood clotting, blood circulation to and from the heart, and muscle movement
Roy
what is muscle tissue
Rifat Reply
the muscle tissue is one of the 4 basic tissues in our body and is responsible for movement of our body
What Is difference between Ovary and Ovum
Sawda
ovary is female reproductive organ while ovum is a female gamete formed from ovary
Ubaidah
Thank You
Sawda
What Is Cholesterol and Triglycerides
Sawda
the mineral salt which break up a large portion of bone tissue
Aurora
Hi Be Home Be safe , how are things doing hope all is well
Natarajan Reply
3 longitudinal bands of smooth muscles found in large intestines
Jamia
what's is sutures
Nimeshka Reply
what would I like to know
Roy Reply
anything u can tell me
Roy
anatomy mins
Manish
when two or more bones meet.
Joseph
Joints
Anita
I am interested in learning but it is a little threatening corona virus covid 19
Samnang Reply
I don't know about Corona virus
Vicky
what would you like to know?
Mbasa
what is a peripheral protien
Ayesha Reply
actually its located in between the lipid layer, it does not specify if it's closer to the inside or the outside of the cell
Justin
It is protein found in lipid bilayer but found attached with Cytoplasm aspect
Jamal
what are the collection of blood.?
sunshine Reply
Effect of exercise on different body systems?
Rania Reply
what is ambroylogy
kashif Reply
embryology..is the biological studing of embryos
Ava
I know biological study but embryology mean any pic, example?
kashif
https://upload.wikimedia.org/wikipedia/commons/b/b6/Comparative_embryology_of_the_vertebrates%3B_with_2057_drawings_and_photos._grouped_as_380_illus_%281953%29_%2820482505100%29.jpg
Ava
I like to learn about medical and more
Samnang
what is the function of the blood
Yolanda Reply
Transporting of oxygen,fighting against germs, forms clotting ,distribution of nutrients and minerals through out the body ,
Nimco
Transportation of gases such as oxygen and water blance and carrei metabolites to the exit organ and Acid base equilibrium and clotting blood and Immune
Jamal
What are Gross and microscopicAnatomy
Waiswa Reply
study of the internal structures of a human being
Niyi
gross anatomy is the study of body parts that can be seen with our naked eyes while micro anatomy involves the study of body parts that cannot be seen with our naked eyes but with the aid of a microscope
Oppong
gross means examination of specimen or tissue with bare (unaided ) eye while microscopic means examination of same with the help of microscope
Jamia

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Source:  OpenStax, Anatomy & Physiology. OpenStax CNX. Feb 04, 2016 Download for free at http://legacy.cnx.org/content/col11496/1.8
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