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Although each complement activation pathway is initiated in a different way, they all provide the same protective outcomes: opsonization, inflammation, chemotaxis , and cytolysis. The term opsonization refers to the coating of a pathogen by a chemical substance (called an opsonin ) that allows phagocytic cells to recognize, engulf, and destroy it more easily. Opsonins from the complement cascade include C1q, C3b, and C4b. Additional important opsonins include mannose-binding proteins and antibodies. The complement fragments C3a and C5a are well-characterized anaphylatoxins with potent proinflammatory functions. Anaphylatoxins activate mast cells, causing degranulation and the release of inflammatory chemical signals, including mediators that cause vasodilation and increased vascular permeability. C5a is also one of the most potent chemoattractants for neutrophils and other white blood cells, cellular defenses that will be discussed in the next section.

The complement proteins C6, C7, C8, and C9 assemble into a membrane attack complex (MAC) , which allows C9 to polymerize into pores in the membranes of gram-negative bacteria. These pores allow water, ions, and other molecules to move freely in and out of the targeted cells, eventually leading to cell lysis and death of the pathogen ( [link] ). However, the MAC is only effective against gram-negative bacteria; it cannot penetrate the thick layer of peptidoglycan associated with cell walls of gram-positive bacteria. Since the MAC does not pose a lethal threat to gram-positive bacterial pathogens, complement-mediated opsonization is more important for their clearance.

Cytokines

Cytokines are soluble proteins that act as communication signals between cells. In a nonspecific innate immune response, various cytokines may be released to stimulate production of chemical mediators or other cell functions, such as cell proliferation, cell differentiation, inhibition of cell division, apoptosis, and chemotaxis.

When a cytokine binds to its target receptor, the effect can vary widely depending on the type of cytokine and the type of cell or receptor to which it has bound. The function of a particular cytokine can be described as autocrine, paracrine, or endocrine ( [link] ). In autocrine function , the same cell that releases the cytokine is the recipient of the signal; in other words, autocrine function is a form of self-stimulation by a cell. In contrast, paracrine function involves the release of cytokines from one cell to other nearby cells, stimulating some response from the recipient cells. Last, endocrine function occurs when cells release cytokines into the bloodstream to be carried to target cells much farther away.

Cytokines are molecular messengers. In autocrine signaling the same cell secretes and receives cytokine signals. The diagram shows a single cell releasing molecules and having the molecules bind to receptors on its surface. In paracrine signaling cytokine signals are secreted to a nearby cell. The diagram shows a cell labeled secreting cell secreting cytokines. A nearby cell has receptors for the molecules. In endocrine signaling cytokine signals are secreted to the circulatory system and travel to distant cells. The diagram shows the secreting cell secreting cytokines; the cytokines then travel through a blood vessel and bind to receptors on a distant cell.
Autocrine, paracrine, and endocrine actions describe which cells are targeted by cytokines and how far the cytokines must travel to bind to their intended target cells’ receptors.

Three important classes of cytokines are the interleukins, chemokines, and interferons. The interleukins were originally thought to be produced only by leukocytes (white blood cells) and to only stimulate leukocytes, thus the reasons for their name. Although interleukins are involved in modulating almost every function of the immune system, their role in the body is not restricted to immunity. Interleukins are also produced by and stimulate a variety of cells unrelated to immune defenses.

Questions & Answers

what a tropism in host
Khaliil Reply
HPV vaccine given to school children
Jayani Reply
Differentiation between electron, proton and neutron
Zainab Reply
proton .possitive charge electron . negative charge neutron . having no charge
Kifayat
proton positive charge. electron negative charge. And no charge of the neutron.
Saman
the nucleus is composed of electrons (-) charge and they turn around the Nucleon the Nucleon = neutron(no charge) + proton (+) a neutron can turn to a proton and vice versa (cuz they have the same mass=1)
BENNINI
what are the roles of microorganisms in human being
Buhari Reply
some causes disease, others are not disease causing
Clark
they're necessary in our digestive system+the skin,everywhere actually the number of them in the human body alone is higher(by millions)than the number of humans cells,they're indispensable in the food industry,others are fundamental to make medicines and more,what exactly are you asking about ?
BENNINI
What are the natural occurring elements found in organisms on earth?
Otu Reply
some of the naturally occurring elements found in organisms are carbon, nitrogen, hydrogen, oxygen
Yashi
thanks
Mariam
some of them are carbon, oxygen, nitrogen, hydrogen, phosphorus and sulfur
Jamal
what is Zika virus?
Somali Reply
Zika virus is a mosquito-borne flavivirus that was first identified in Uganda in 1947 in monkeys. 
Cumar
in pregnancy it can cause the unborn child's head to underdevelop so that it does not grow along with the rest of the body
Bethany
intracellular vesicles are found in
Akshay Reply
how do i report widal slide results
Isaac Reply
how food can be used as a substrate for the growth of microorganisms
Ashi Reply
what is the importance of understanding chemistry in the field of microbiology and food technology?
Charmaine Reply
What is different between eukaryotes and prokcaryotic
Abia Reply
main difference is that eukaryotic cells possess membrane bound organelles
Iqra
prokaryotes are primitive organisms that doesn't contain membrane bound nucleus or any orgenelle while in eukaryotes membrane bound orgenelles and nucleus is present
Yashi
what s anatomy
jane Reply
its mean body structure, function, and systems
gazi
study of internal structure of living things
Falere
anatomy is the study structure of the made human
isir
anotomy is the actual study of body internally and externally Which include how itis made. for what and what is need of this
Iqra
la science anatomie c'est la science qui nous aide à étudier l'homme
Baraka
okay
Baraka
Ancestor are they real
Rapheal Reply
yes of course
David
yes please
Clark
50 50, depending on the accuracy of the clan records.
Vincent
okay
Baraka
Why protist is not a kingdom of Linnaeus 'S taxonomy?
Neha Reply
I dont know
Jeewraj
Linnaeus used Aristotle's criteria for dividing living organisms into kingdoms. Plants are immobile and insensitive, while animals are mobile and sensitive. the creatures he saw through the microscope were mobile, so he attributed them to animals.
Lad
name the different types of media use in lab to detect the micro organisms
Shehzadi Reply
The different types of media used in the lab to detect the microorganisms is known as cell culture..
Tean
solid media or broth is used to grow and detect microorganisms
Pohor
what is microbiology
Baraka
the study of large living organisms
Pratibha
the study of organisms which are micro in range
Vency
what is autoclaving?
Yashi
process for sterilization
Vency
is a machine used on the process of sterilisation
Clark
what is difference between hot air oven and autoclave as they both are used for sterilization ?
Vency
autoclave basically do moist heat sterilization while hot air oven do sterilization by dry heat.....
Yashi
what is the microbe
HUSSAIN
which method out of these two is best?
Yashi
why human have microbe
HUSSAIN
Hi
Sadam
hi to all
Sadam
depends on what to sterilize
Vency
microbes
Md
what is knowledge
HUSSAIN
well idea
Daniel
Practice MCQ 5

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Source:  OpenStax, Microbiology. OpenStax CNX. Nov 01, 2016 Download for free at http://cnx.org/content/col12087/1.4
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