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Immunity B-lymphocytes activation help??

Hey. I do AQA Biology and am currently in Year 12, today I have spent hours and hours trying to properly understand immunity, the cell-mediated response, and the humoral response and I'm STILL confused.

I will try and summarise what I'm most confused about.
So, B-lymphocytes are activated, and subsequently they produce memory-B cells, which circulate the bloodstream and become activated on a repeat infection; and plasma cells, which secrete antibodies.
I'm struggling to understand how exactly they are activated - I've had it from several different sources that:

a) Helper-T cells release cytokines, once they (t cells) have been activated by antigens presented by phagocytes. These cytokines stimulate the rapid division of B-lymphocytes into clones, which then differentiate into plasma and memory cells.

b) Activated Helper-T cells present the antigens of the pathogen to the antibody of the B-lymphocyte, stimulating it to divide and differentiate etc

c) B-lymphocyte engulfs free antigens from the blood and then presents them, enabling a T-helper cell to bind to it and thus activate the B-lymphocyte, leading to division/differentiation etc

Which of these is correct? Is more than one correct? Is there anything I have missed out?
I really hope someone can answer and help me to understand this because I've been struggling with it for ages. :/
Reply 1
Original post by ConfusedSnowball
Hey. I do AQA Biology and am currently in Year 12, today I have spent hours and hours trying to properly understand immunity, the cell-mediated response, and the humoral response and I'm STILL confused.

I will try and summarise what I'm most confused about.
So, B-lymphocytes are activated, and subsequently they produce memory-B cells, which circulate the bloodstream and become activated on a repeat infection; and plasma cells, which secrete antibodies.
I'm struggling to understand how exactly they are activated - I've had it from several different sources that:

a) Helper-T cells release cytokines, once they (t cells) have been activated by antigens presented by phagocytes. These cytokines stimulate the rapid division of B-lymphocytes into clones, which then differentiate into plasma and memory cells.

b) Activated Helper-T cells present the antigens of the pathogen to the antibody of the B-lymphocyte, stimulating it to divide and differentiate etc


All right except this:
c) B-lymphocyte engulfs free antigens from the blood and then presents them, enabling a T-helper cell to bind to it and thus activate the B-lymphocyte, leading to division/differentiation etc/


A specific B-lympocyte will recognise a free floating, non processed specific antigen with its receptor(s), but the binding itself will cause clonal expansion and activation. Helper T cells can stimulate B-cells though, making them "more" activated.
Original post by h3rmit
All right except this:


A specific B-lympocyte will recognise a free floating, non processed specific antigen with its receptor(s), but the binding itself will cause clonal expansion and activation. Helper T cells can stimulate B-cells though, making them "more" activated.


Thank you for your reply! So would I be correct in saying that b-lymphocytes are stimulated by contact with either a processed (from a t-helper cell) or non-processed (free floating) specific antigen which binds to their antibody, but they can additionally be stimulated by cytokines released from helper-t cells?


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Original post by ConfusedSnowball
Hey. I do AQA Biology and am currently in Year 12, today I have spent hours and hours trying to properly understand immunity, the cell-mediated response, and the humoral response and I'm STILL confused.

I will try and summarise what I'm most confused about.
So, B-lymphocytes are activated, and subsequently they produce memory-B cells, which circulate the bloodstream and become activated on a repeat infection; and plasma cells, which secrete antibodies.
I'm struggling to understand how exactly they are activated - I've had it from several different sources that:

a) Helper-T cells release cytokines, once they (t cells) have been activated by antigens presented by phagocytes. These cytokines stimulate the rapid division of B-lymphocytes into clones, which then differentiate into plasma and memory cells.

b) Activated Helper-T cells present the antigens of the pathogen to the antibody of the B-lymphocyte, stimulating it to divide and differentiate etc

c) B-lymphocyte engulfs free antigens from the blood and then presents them, enabling a T-helper cell to bind to it and thus activate the B-lymphocyte, leading to division/differentiation etc

Which of these is correct? Is more than one correct? Is there anything I have missed out?
I really hope someone can answer and help me to understand this because I've been struggling with it for ages. :/


Let me explain using a summary I made last year (1st year of med school). B cells are activated in 2 ways:

- T-cell independent activation = A B-cell is activated directly by an antigen and proceeds to release IgM antibodies
- T-cell dependent activation = This is where cells called dendritic cells phagocytose the antigen and present it on its cell surface using MHC II molecules, which are recognised by Th2 CD4 cells (T-helper cells). These T-helper cells become activated and when they meet a B cell presenting the same antigen then they activate that B cell. There is always co-stimulation in these processes.

Hopefully that answers your question a little, if it doesn't at all then let me know and I can make a better attempt at explaining it simply haha

Note: I edited it because I got something a little wrong, I hope everything above is correct now!
(edited 7 years ago)
Original post by AortaStudyMore
Let me explain using a summary I made last year (1st year of med school). B cells are activated in 2 ways:

- T-cell independent activation = A B-cell is activated directly by an antigen and proceeds to release IgM antibodies
- T-cell dependent activation = This is where cells called dendritic cells phagocytose the antigen and present it on its cell surface using MHC II molecules, which are recognised by Th2 CD4 cells (T-helper cells). These T-helper cells become activated and when they meet a B cell presenting the same antigen then they activate that B cell. There is always co-stimulation in these processes.

Hopefully that answers your question a little, if it doesn't at all then let me know and I can make a better attempt at explaining it simply haha

Note: I edited it because I got something a little wrong, I hope everything above is correct now!


Ah okay, and where would the cytokines come in?
For a T-helper cell to activate a B cell, must the B cell be presenting an antigen they acquired through T-cell independent activation?
Can a B cell be activated by contact with a pathogen presenting its own antigens?
And what do you mean by 'co-stimulation'?

Sorry for all the questions haha. And btw, I love your username!
Original post by ConfusedSnowball
Ah okay, and where would the cytokines come in?
For a T-helper cell to activate a B cell, must the B cell be presenting an antigen they acquired through T-cell independent activation?
Can a B cell be activated by contact with a pathogen presenting its own antigens?
And what do you mean by 'co-stimulation'?

Sorry for all the questions haha. And btw, I love your username!


No don't worry about the questions! Let me explain a bit better:

B cells and dendritic cells are known as antigen presenting cells (APCs), this means that they phagocytose antigens and present a bit of it on their surface (known as the "epitope" I think). These dendritic cells and B cells then migrate to the lymph nodes. The dendritic cells present the antigen on its surface which is then bound to by a T-helper cell. The T-helper cell is now active and begins searching for a B cell that is also presenting the same antigen on its surface (because remember that B cells are also APCs), if the T cell finds a B cell presenting the antigen, then it will bind and activate the B cell to begin forming plasma cells (and memory cells).

The co-stimulation I referred to will be the release of cytokines when a receptor binds to an antigen being presented on a cell, you need the receptor binding AND the stimulation by cytokines for activation to occur. Cytokines are released all over the place in immune responses, in particular interleukins and interferons. So in answer to your original question, B cells are activated by T cell dependent activation when they present an antigen on their surface from phagocytosis and are then bound to by a T helper cell.

As for your second question, as I said before, B cells CAN be activated directly by the antigen (T cell independent activation), but as far as I know this only occurs with specfic antigens.

Anyway I hope that helps, my immunology is a bit shaky as it's been just under a year since I revised it all, and it's a complicated topic, I tried summarising the whole immune resonse on an A4 page last year, it's really good but very confusing!
Original post by ConfusedSnowball
Ah okay, and where would the cytokines come in?
For a T-helper cell to activate a B cell, must the B cell be presenting an antigen they acquired through T-cell independent activation?
Can a B cell be activated by contact with a pathogen presenting its own antigens?
And what do you mean by 'co-stimulation'?

Sorry for all the questions haha. And btw, I love your username!


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Here you go, here's my summary if you're interested (you obviously won't need to know this much detail for AS level, but if you're into medicine/immunity then it might be a little interesting). But don't worry if you can't understand most of it, because I don't either and I made it..
Reply 7
Original post by AortaStudyMore
The T-helper cell is now active and begins searching for a B cell that is also presenting the same antigen on its surface (because remember that B cells are also APCs), if the T cell finds a B cell presenting the antigen, then it will bind and activate the B cell to begin forming plasma cells (and memory cells).


Shouldn't this be if the B cell has a receptor that's complementary to the epitope, as wouldn't the B-cell already have been activated if it was bound to its complementary antigen?
Original post by h3rmit
Shouldn't this be if the B cell has a receptor that's complementary to the epitope, as wouldn't the B-cell already have been activated if it was bound to its complementary antigen?


The BCR binds to the antigen and takes up the antigen before processing it and presenting it on MHCII molecules. The T-helper cells have CD4 molecules and so bind to cells presenting MHCII, ie the B cells. Once bound, the T cell releases cytokines to stimulate the production of transcription factors in the B cell that allows it to differentiate into a plasma cell. The B cell is activated by the T cell, which was originally activated by the dendritic cell. As my diagram shows, I think follicular dendritic cells can directly activate B cells, but I never really got around the mechanism of that when revising for my exam last year.

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