Bertybassett
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Hi there, I am feeling very confused in regards to immunity and the process that occurs when antigens enter the blood. I feel like I get different answers in every textbook or youtube video I watch. Is it the T or B cells that initially bind to the antigen (which I assume if present on an antigen presenting cell?), as one of my textbooks says that it is the T helper cells, and these then activate B cells to bind, activate cytotoxic t cells etc? Yet in a markscheme it said that B cells bind, then undergo mitosis which forms plasma cells etc? What is the general process after this? When the cells do attach, is it to "free floating" bacteria in the blood or to antigens present as a result of phagosytosis on antigen presenting cells? Finally, is it the antigens on the surface of b cells or receptors that bind to the antigen? Many thanks
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Study Stream
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Firstly, I am fairly sure that it is both the T cells and the B cells which can bind to the antigen-presenting cell or the antigen on a bacterial cell, as they both have receptors on their surface which is complementary to the antigen (the B-cell has antibodies and the T-cell has a general receptor), so they both initiate colonal selection. However, it is important to note that the T-cells most likely have a prodominant role as the t-helped cells can release cytokines to cause colonal selection and expansion of the B-cells.

Secondly, Following plasma cell production, they will start to secrete loads of antibodies, specfic to the antigen, to start battling the infection via aggulutinisation, neutralisation and opsonin action.

Thirdly, they can attach to both the bacteria antigen and antigen on the phagocyte.

Fourthly, the antigens on the b-cell are only there for self-recognition and will not be complementary to the bacterial antigen- therefore it is important to state that the antibody/receptor binds to the antigen.
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The A-level kid
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(Original post by Bertybassett)
Hi there, I am feeling very confused in regards to immunity and the process that occurs when antigens enter the blood. I feel like I get different answers in every textbook or youtube video I watch. Is it the T or B cells that initially bind to the antigen (which I assume if present on an antigen presenting cell?), as one of my textbooks says that it is the T helper cells, and these then activate B cells to bind, activate cytotoxic t cells etc? Yet in a markscheme it said that B cells bind, then undergo mitosis which forms plasma cells etc? What is the general process after this? When the cells do attach, is it to "free floating" bacteria in the blood or to antigens present as a result of phagosytosis on antigen presenting cells? Finally, is it the antigens on the surface of b cells or receptors that bind to the antigen? Many thanks
Initially, T-cells bind to the antigens present on an Antigen Presenting cell (APC). Remember this very important point: The difference between T-cells and B-cells is that T-cells can only bind onto antigens on APC's and NOT onto the pathogens themselves like B-cells can. Antigens as you may already know are proteins found on the surface of pathogens but can be presented on the outside of phagocytes when they engulf these pathogens to signal to the T-cells that a foreign body has entered into circulation. T-cells are divided into T-helper (Th), T-Killer (Tk) and T-memory (Tm) cells. T-helper cells 'help' the immune system by activating Tk cells and naive B cells. Naive meaning they have never seen an antigen in their lives and are literally new innocent cells who know nothing about the foreign world. So initially, Th cells bind to APC's this triggers clonal selection and expansion (mitosis of Th cells) to produce specific Th cells with the same receptor that can bind to further identical antigens. Th cells release cytokines which activate the naive B cells and Tk cells. Tk cells 'kill' infected cells by releasing cytotoxins and perfornin destroying them. Tm cells "remember" the exact receptor required for that specific antigen so in future, if this pathogen with the same antigens enters the body a second time, the immune system can react faster and more efficiently and therefore remove the pathogens quicker which should theoretically mean a person will have no symptoms. Moving onto the naive B-cells, so, as mentioned, Th cells also activate naive B-cells by binding to the B cell receptors via complementary binding and releasing cytokines. These B-cells undergo clonal selection and expansion via mitosis producing lots more identical B-cells which then differentiate further into B-plasma cells and B-memory cells. B-plasma cells (aka b-effector cells) secrete antibodies which have 3 major functions to aid in destroying pathogens. Please Note: Antibodies can either be fixed on the B-cell or can be secreted into the blood steam on their own. Remember, antibodies are antigen specific and form antigen-antibody complexes. 1) Agglutination - Antibodies surround pathogens and attach to the antigens via its variable region. Phagocytes target the constant regions of antibodies and engulf the entire mixture. In agglutination antibodies "glue" pathogens together causing macrophages to engulf large numbers of pathogens all at once, making it a lot more efficient than killing one pathogen at a time.
2) Antibodies can also attach to toxins and neuturalise them so they are thereafter phagocytosed.
3) Antibodies can opsonize meaning they all surround a pathogen and act as a signal for phagocytes to find these pathogens quicker and engulf them in contrast to searching for pathogens alone. Think of antibodies like witnesses helping the police (phagocytes) find them and get rid of them quicker.
B memory cells have the same function as Tm cells except its to B-cells instead.

Answering your questions specifically
1) T-cells initially bind to antigens on antigen presenting cells, specifically firstly it is T-helper cells. T-helper cells thereafter activate B-cells and Tk cells.
2) Do you have the exam question to the mark scheme answer that you are referring to?
3) The process after B-cell activation as mentioned already is they undergo clonal expansion and either secreted antibodies or antibodies fixed on B-cells go ahead and target pathogens as "witnesses" to the "crime" to help phagocytes find them quicker via the 3 major methods as stated. After all of this, the memory cells remember these receptors in the event that the pathogen comes again another time or in a police analogy, the police have the suspects details etc on file in case they commit other offences in future.
4) Remember, T-cells can only bind to APC's and not to the free floating bacteria directly, only B-cells via antibodies can do so.
5) It is antibodies on B-plasma cells that bind to the antigen. Remember, some B-cells can secrete antibodies instead and therefore lone antibodies can bind to the antigens on pathogens. B-cells do not present antigens as antigens can ONLY be found on pathogens which is what makes pathogens unique (like a suspects fingerprint). The difference between a naive B cell and B plasma cell is that a naive B cell has receptors which binds to Th cells to undergo clonal expansion and differentiate into B plasma and B memory cells but a B plasma cell has antibodies on its surface, not receptors.

I hope I have helped you, any questions please don't hesitate to ask!
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Bertybassett
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(Original post by The A-level kid)
Initially, T-cells bind to the antigens present on an Antigen Presenting cell (APC). Remember this very important point: The difference between T-cells and B-cells is that T-cells can only bind onto antigens on APC's and NOT onto the pathogens themselves like B-cells can. Antigens as you may already know are proteins found on the surface of pathogens but can be presented on the outside of phagocytes when they engulf these pathogens to signal to the T-cells that a foreign body has entered into circulation. T-cells are divided into T-helper (Th), T-Killer (Tk) and T-memory (Tm) cells. T-helper cells 'help' the immune system by activating Tk cells and naive B cells. Naive meaning they have never seen an antigen in their lives and are literally new innocent cells who know nothing about the foreign world. So initially, Th cells bind to APC's this triggers clonal selection and expansion (mitosis of Th cells) to produce specific Th cells with the same receptor that can bind to further identical antigens. Th cells release cytokines which activate the naive B cells and Tk cells. Tk cells 'kill' infected cells by releasing cytotoxins and perfornin destroying them. Tm cells "remember" the exact receptor required for that specific antigen so in future, if this pathogen with the same antigens enters the body a second time, the immune system can react faster and more efficiently and therefore remove the pathogens quicker which should theoretically mean a person will have no symptoms. Moving onto the naive B-cells, so, as mentioned, Th cells also activate naive B-cells by binding to the B cell receptors via complementary binding and releasing cytokines. These B-cells undergo clonal selection and expansion via mitosis producing lots more identical B-cells which then differentiate further into B-plasma cells and B-memory cells. B-plasma cells (aka b-effector cells) secrete antibodies which have 3 major functions to aid in destroying pathogens. Please Note: Antibodies can either be fixed on the B-cell or can be secreted into the blood steam on their own. Remember, antibodies are antigen specific and form antigen-antibody complexes. 1) Agglutination - Antibodies surround pathogens and attach to the antigens via its variable region. Phagocytes target the constant regions of antibodies and engulf the entire mixture. In agglutination antibodies "glue" pathogens together causing macrophages to engulf large numbers of pathogens all at once, making it a lot more efficient than killing one pathogen at a time.
2) Antibodies can also attach to toxins and neuturalise them so they are thereafter phagocytosed.
3) Antibodies can opsonize meaning they all surround a pathogen and act as a signal for phagocytes to find these pathogens quicker and engulf them in contrast to searching for pathogens alone. Think of antibodies like witnesses helping the police (phagocytes) find them and get rid of them quicker.
B memory cells have the same function as Tm cells except its to B-cells instead.

Answering your questions specifically
1) T-cells initially bind to antigens on antigen presenting cells, specifically firstly it is T-helper cells. T-helper cells thereafter activate B-cells and Tk cells.
2) Do you have the exam question to the mark scheme answer that you are referring to?
3) The process after B-cell activation as mentioned already is they undergo clonal expansion and either secreted antibodies or antibodies fixed on B-cells go ahead and target pathogens as "witnesses" to the "crime" to help phagocytes find them quicker via the 3 major methods as stated. After all of this, the memory cells remember these receptors in the event that the pathogen comes again another time or in a police analogy, the police have the suspects details etc on file in case they commit other offences in future.
4) Remember, T-cells can only bind to APC's and not to the free floating bacteria directly, only B-cells via antibodies can do so.
5) It is antibodies on B-plasma cells that bind to the antigen. Remember, some B-cells can secrete antibodies instead and therefore lone antibodies can bind to the antigens on pathogens. B-cells do not present antigens as antigens can ONLY be found on pathogens which is what makes pathogens unique (like a suspects fingerprint). The difference between a naive B cell and B plasma cell is that a naive B cell has receptors which binds to Th cells to undergo clonal expansion and differentiate into B plasma and B memory cells but a B plasma cell has antibodies on its surface, not receptors.

I hope I have helped you, any questions please don't hesitate to ask!
Hi, thanks so much for the detailed reply. I really appreciate the help. I have a few questions regarding the process. Firstly, I understand that T cells can only bind to AP cells, but are B cells able to bind to APCs also or can they only bind to the free floating pathogens? Furthermore, when the b cells have been activated by the t cells, do they bind to the same cell that the t cell has binded to or do they just bind to other pathogens with the same antigen? Also, what did you mean by t cells release cytokines - what are these and what do they do? Finally, what do you mean by clonal selection?

Many thanks for all of your help.
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The A-level kid
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Hi,
B-cells cannot bind to APCs, they only bind to Th cells initially and then to antigens on pathogens directly via antibodies (when specialised).
When the b cell has been activated by Th cell, no it does not bind to the one that Th is bound to as b cells do not target APCs. That cell will most likely be killed by Tk cell. Cytokines are particular types of protein released by Th cells which act as a signal to other cells of the immune system to target a particular antigen. It is how the cell communicates to other cells that a foreign pathogen has entered the system. Cytokines consist of growth factors, interleukins etc but I don't think you need that much detail for A-level.
Clonal selection simply means the stage where the Th cell or B cell has "selected" the cell it has found and matched with and now undergoes "clonal" expansion where it duplicates itself via mitosis. E.g. When the exact specific Th cell finds the matching APC via its antigens this is clonal selection.
(Original post by Bertybassett)
Hi, thanks so much for the detailed reply. I really appreciate the help. I have a few questions regarding the process. Firstly, I understand that T cells can only bind to AP cells, but are B cells able to bind to APCs also or can they only bind to the free floating pathogens? Furthermore, when the b cells have been activated by the t cells, do they bind to the same cell that the t cell has binded to or do they just bind to other pathogens with the same antigen? Also, what did you mean by t cells release cytokines - what are these and what do they do? Finally, what do you mean by clonal selection?

Many thanks for all of your help.
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Bertybassett
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thanks for the reply. is it the cytokines that activate the cytotoxic t cells, and do they activate the b cells to bind to the antigen? What else do they do? thanks
(Original post by The A-level kid)
Hi,
B-cells cannot bind to APCs, they only bind to Th cells initially and then to antigens on pathogens directly via antibodies (when specialised).
When the b cell has been activated by Th cell, no it does not bind to the one that Th is bound to as b cells do not target APCs. That cell will most likely be killed by Tk cell. Cytokines are particular types of protein released by Th cells which act as a signal to other cells of the immune system to target a particular antigen. It is how the cell communicates to other cells that a foreign pathogen has entered the system. Cytokines consist of growth factors, interleukins etc but I don't think you need that much detail for A-level.
Clonal selection simply means the stage where the Th cell or B cell has "selected" the cell it has found and matched with and now undergoes "clonal" expansion where it duplicates itself via mitosis. E.g. When the exact specific Th cell finds the matching APC via its antigens this is clonal selection.
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Bertybassett
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Hi there, thanks again. On this paper on question 4.4 I was a bit confused. So I understand from the ms that the th cell binds directly to the tumour cell which in this case is the antigen presenting cell. But if this is the case, what is the point in phagocytosis - surely the th cell can then bind directly to the bacteria or virus instead of to the phagocyte that acts as the apc ? Furthermore, I was wondering what you meant by cytokines? What do they actually do. Many thanks https://filestore.aqa.org.uk/resourc...-74023-SQP.PDF
(Original post by The A-level kid)
Initially, T-cells bind to the antigens present on an Antigen Presenting cell (APC). Remember this very important point: The difference between T-cells and B-cells is that T-cells can only bind onto antigens on APC's and NOT onto the pathogens themselves like B-cells can. Antigens as you may already know are proteins found on the surface of pathogens but can be presented on the outside of phagocytes when they engulf these pathogens to signal to the T-cells that a foreign body has entered into circulation. T-cells are divided into T-helper (Th), T-Killer (Tk) and T-memory (Tm) cells. T-helper cells 'help' the immune system by activating Tk cells and naive B cells. Naive meaning they have never seen an antigen in their lives and are literally new innocent cells who know nothing about the foreign world. So initially, Th cells bind to APC's this triggers clonal selection and expansion (mitosis of Th cells) to produce specific Th cells with the same receptor that can bind to further identical antigens. Th cells release cytokines which activate the naive B cells and Tk cells. Tk cells 'kill' infected cells by releasing cytotoxins and perfornin destroying them. Tm cells "remember" the exact receptor required for that specific antigen so in future, if this pathogen with the same antigens enters the body a second time, the immune system can react faster and more efficiently and therefore remove the pathogens quicker which should theoretically mean a person will have no symptoms. Moving onto the naive B-cells, so, as mentioned, Th cells also activate naive B-cells by binding to the B cell receptors via complementary binding and releasing cytokines. These B-cells undergo clonal selection and expansion via mitosis producing lots more identical B-cells which then differentiate further into B-plasma cells and B-memory cells. B-plasma cells (aka b-effector cells) secrete antibodies which have 3 major functions to aid in destroying pathogens. Please Note: Antibodies can either be fixed on the B-cell or can be secreted into the blood steam on their own. Remember, antibodies are antigen specific and form antigen-antibody complexes. 1) Agglutination - Antibodies surround pathogens and attach to the antigens via its variable region. Phagocytes target the constant regions of antibodies and engulf the entire mixture. In agglutination antibodies "glue" pathogens together causing macrophages to engulf large numbers of pathogens all at once, making it a lot more efficient than killing one pathogen at a time.
2) Antibodies can also attach to toxins and neuturalise them so they are thereafter phagocytosed.
3) Antibodies can opsonize meaning they all surround a pathogen and act as a signal for phagocytes to find these pathogens quicker and engulf them in contrast to searching for pathogens alone. Think of antibodies like witnesses helping the police (phagocytes) find them and get rid of them quicker.
B memory cells have the same function as Tm cells except its to B-cells instead.

Answering your questions specifically
1) T-cells initially bind to antigens on antigen presenting cells, specifically firstly it is T-helper cells. T-helper cells thereafter activate B-cells and Tk cells.
2) Do you have the exam question to the mark scheme answer that you are referring to?
3) The process after B-cell activation as mentioned already is they undergo clonal expansion and either secreted antibodies or antibodies fixed on B-cells go ahead and target pathogens as "witnesses" to the "crime" to help phagocytes find them quicker via the 3 major methods as stated. After all of this, the memory cells remember these receptors in the event that the pathogen comes again another time or in a police analogy, the police have the suspects details etc on file in case they commit other offences in future.
4) Remember, T-cells can only bind to APC's and not to the free floating bacteria directly, only B-cells via antibodies can do so.
5) It is antibodies on B-plasma cells that bind to the antigen. Remember, some B-cells can secrete antibodies instead and therefore lone antibodies can bind to the antigens on pathogens. B-cells do not present antigens as antigens can ONLY be found on pathogens which is what makes pathogens unique (like a suspects fingerprint). The difference between a naive B cell and B plasma cell is that a naive B cell has receptors which binds to Th cells to undergo clonal expansion and differentiate into B plasma and B memory cells but a B plasma cell has antibodies on its surface, not receptors.

I hope I have helped you, any questions please don't hesitate to ask!
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