Then all of a sudden, that bacteria is going to be completely engulfed. It's going to be inside of the cell. So now the cell-- once these two ends meet and these membranes merge, then this guy is going to be in his own little membrane bubble-- or you can almost imagine, it's in its own little vesicle. So this is the pathogen, the bacteria in this case-- but phagocytosis-- the process is completely identical in terms of how it engulfs things.
If it was a virus or some type of other foreign protein or any type of really foreign molecule-- actually, sometimes it doesn't even occur to foreign stuff. It can occur to dying molecules that are not foreign, that just need to be cleared out. But we'll just focus on the immune system, on foreign things right now. So this membrane right here will completely merge and go around this guy like this. And of course, you had your receptors and who knows if they're still there. By the time-- let's just draw them there so you see that that part is that part.
But once it's fully engulfed, this thing is called a phagosome, on which is really just a vesicle that contains that foreign particle that you want to get rid of. And then other fluid or vesicles that contain things that can eat up this phagosome-- so let's say that this is some vesicle that contains things-- lysozymes and it contains really reactive species of oxygen.
And if this comes in contact with, really, almost any biological compound, it's going to do some damage. But once the pathogen is completely merged inside the cell, this little package will merge over here and it will dump its contents into this phagosome, into this vesicle containing the pathogen, and then break it up.
It's essentially digesting it. So obviously the first role is, it just got it out of the way and it killed it. And then the second role-- and I'm just going to give a little tidbit right here. We're going to do it in a lot more detail in future videos. It breaks it up. So now the thing is all broken up. So that thing is broken up into constituent proteins and another molecules. And then what the phagocyte does-- it'll actually take some subset of these molecules, some subset of the proteins.
It'll break them out. Proteins are just sequences of amino acids. Normally when people say proteins, they're talking about long sequences of amino acids. When people talk about short sequences of amino acids or a protein that's broken up a lot, they refer to it as a peptide chain.
A peptide chain is a shorter chain of amino acids. So this guy will take some special peptide chains, some special pieces from the thing it just killed, attach them to some other proteins. So it'll take maybe a little piece of this bacteria right now, attach it to other protein, which is called a major histocompatibility complex-- and if we're talking about phagocytes, this will be a major histocompatibility complex type II.
It sounds very-- a strange word, but we're going to see this a lot. So they abbreviate it MHC. This is a protein and it bonds with this peptide that was kind of chunked off or digested off of this invading pathogen and then this phagocyte will then present it onto its membrane.
We're going to talk about Type I in the future. It's going to take this complex and then present it on its surface. And the reason why I'm going through all this pain of explaining this process-- you're like, hey, we already got rid of the thing and killed it.
Why is Sal worried about what we do with the peptides? This is crucial to our immune system because we'll see other specific parts of our immune system. Remember, so far everything is non-specific. This guy just said, this is an invader. It doesn't know the type of an invader.
It just says, hey, let me bond to this thing and kill it. It's one of these things that I know are foreign to my body.
So it kills it, but now it can leave it on its surface and now the specific parts, the parts that actually have memory and attack specific things, can say, gee, Mr. Phagocyte, look, you've killed something. Let me see if I have some specific reactions that can be triggered by this thing that you're presenting. So, many phagocytes are also called antigen presenting cells. And I'm going to go into more detail on what exactly an antigen is.
I called this thing a pathogen. An antigen is essentially-- you can view it as a protein or a peptide chain that will trigger or that can be dealt with within the immune system. Infections cause serious and sometimes disfiguring sores and ulcers in the skin and other tissues Figure Worldwide, an estimated 1.
Salivary peptides from the sand fly activate host macrophages at the site of their bite. The classic or alternate pathway for complement activation ensues with C3b opsonization of the parasite. Leishmania cells are phagocytosed, lose their flagella, and multiply in a form known as an amastigote Leishman-Donovan body within the phagolysosome.
Although many other pathogens are destroyed in the phagolysosome, survival of the Leishmania amastigotes is maintained by the presence of surface lipophosphoglycan and acid phosphatase. These substances inhibit the macrophage respiratory burst and lysosomal enzymes.
The parasite then multiplies inside the cell and lyses the infected macrophage, releasing the amastigotes to infect other macrophages within the same host. Should another sand fly bite an infected person, it might ingest amastigotes and then transmit them to another individual through another bite. There are several different forms of leishmaniasis. The most common is a localized cutaneous form of the illness caused by L. A mucocutaneous form of the disease, caused by L.
A visceral form of the illness can be caused by several of the different Leishmania species. It affects various organ systems and causes abnormal enlargement of the liver and spleen. Irregular fevers, anemia, liver dysfunction, and weight loss are all signs and symptoms of visceral leishmaniasis. If left untreated, it is typically fatal. As an Amazon Associate we earn from qualifying purchases. Want to cite, share, or modify this book?
This book is Creative Commons Attribution License 4. Skip to Content Go to accessibility page. Microbiology My highlights. Table of contents. Review Questions. Answer Key. Learning Objectives By the end of this section, you will be able to: Explain how leukocytes migrate from peripheral blood into infected tissues Explain the mechanisms by which leukocytes recognize pathogens Explain the process of phagocytosis and the mechanisms by which phagocytes destroy and degrade pathogens Several of the cell types discussed in the previous section can be described as phagocytes—cells whose main function is to seek, ingest, and kill pathogens.
Extravasation Diapedesis of Leukocytes Some phagocytes are leukocytes WBCs that normally circulate in the bloodstream. Figure In addition, activation of complement at the site of infection results in production of the chemotactic and proinflammatory C5a. Leukocytes exit the blood vessel and follow the chemoattractant signal of cytokines and C5a to the site of infection. Granulocytes such as neutrophils release chemicals that destroy pathogens.
They are also capable of phagocytosis and intracellular killing of bacterial pathogens. Only vertebrates have specific immune responses. Two types of white blood cells called lymphocytes are vital to the specific immune response. Lymphocytes are produced in the bone marrow, and mature into one of several subtypes. The two most common are T cells and B cells. An antigen is a foreign material that triggers a response from T and B cells. The human body has B and T cells specific to millions of different antigens.
We usually think of antigens as part of microbes, but antigens can be present in other settings. For example, if a person received a blood transfusion that did not match his blood type, it could trigger reactions from T and B cells.
A useful way to think of T cells and B cells is as follows: B cells have one property that is essential. They can mature and differentiate into plasma cells that produce a protein called an antibody.
This protein is specifically targeted to a particular antigen. However, B cells alone are not very good at making antibody and rely on T cells to provide a signal that they should begin the process of maturation. When a properly informed B cell recognizes the antigen it is coded to respond to, it divides and produces many plasma cells.
The plasma cells then secrete large numbers of antibodies, which fight specific antigens circulating in the blood. T cells are activated when a particular phagocyte known as an antigen-presenting cell APC displays the antigen to which the T cell is specific.
This blended cell mostly human but displaying an antigen to the T cell is a trigger for the various elements of the specific immune response. A subtype of T cell known as a T helper cell performs a number of roles. T helper cells release chemicals to. Regulatory T cells also called suppressor T cells help to control the immune response.
They recognize when a threat has been contained and then send out signals to stop the attack. The cells that make up the specific immune response circulate in the blood, but they are also found in a variety of organs.
Within the organ, immune tissues allow for maturation of immune cells, trap pathogens and provide a place where immune cells can interact with one another and mount a specific response. Infection occurs when a pathogen invades body cells and reproduces. Infection will usually lead to an immune response. If the response is quick and effective, the infection will be eliminated or contained so quickly that the disease will not occur.
Sometimes infection leads to disease. Here we will focus on infectious disease, and define it as a state of infection that is marked by symptoms or evidence of illness. Disease can occur when immunity is low or impaired, when virulence of the pathogen its ability to damage host cells is high, and when the number of pathogens in the body is great.
Depending on the infectious disease, symptoms can vary greatly. Fever is a common response to infection: a higher body temperature can heighten the immune response and provide a hostile environment for pathogens. Inflammation, or swelling caused by an increase in fluid in the infected area, is a sign that white blood cells are on the attack and releasing substances involved in the immune response.
Vaccination works to stimulate a specific immune response that will create memory B and T cells specific to a certain pathogen. These memory cells persist in the body and can lead to a quick and effective response should the body encounter the pathogen again. Hunt, R. Virology: Microbiology and Immunology Online. University of South Carolina. The Merck Manual: Home Edition.
Delves, P. Overview of the Immune System. Article Menu [ ]. Vaccine Science [ ]. Biological Weapons, Bioterrorism, and Vaccines. Cancer Vaccines and Immunotherapy.
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