white_o
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Hi,

So I'm a bit confused about MHC 2 presentation. I'm not sure of where B cells fit into this in terms of clonal selection and expansion.

To my understanding:

Antibodies on B cells recognise antigens from invading pathogens and bind to them which activates B cells. (clonal selection)

It is then taken up by the B cell (phagocytosis?) and part of it is presented on MHC 2.

T cells bind to the MHC 2 - peptide complex,

The B cells can then undergo clonal expansion.

Is this a correct summary?

Thanks!
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Jpw1097
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Report 2 years ago
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(Original post by white_o)
Hi,

So I'm a bit confused about MHC 2 presentation. I'm not sure of where B cells fit into this in terms of clonal selection and expansion.

To my understanding:

Antibodies on B cells recognise antigens from invading pathogens and bind to them which activates B cells. (clonal selection)

It is then taken up by the B cell (phagocytosis?) and part of it is presented on MHC 2.

T cells bind to the MHC 2 - peptide complex,

The B cells can then undergo clonal expansion.

Is this a correct summary?

Thanks!
MHC II molecules are only expressed on the surface of professional antigen presenting cells (APCs) such as dendritic cells, macrophages and B-cells. This is in contrast to MHC I molecules, which present endogenous peptides (i.e. originating from within the cell), which are expressed on the surface of all nucleated cells.

Let's use the B-cell as the APC, as that is what you mentioned. When antigen binds to the B-cell receptor, it is taken into the B-cell via endocytosis and lysosomes then fuse with the phagosome, which degrade the antigen into multiple peptides (phagocytosis). The peptides then associate with MHC II molecules and are transported to the cell surface, where the MHC II: peptide complex is presented to helper T-cells (CD4+ T cells). The T-cell receptor then binds to the MHC II: peptide complex, which activates the helper T cell. The helper T cell then release cytokines which activates the B-cell, causing it to proliferate differentiate into plasma cells (which secrete antibody) or memory B-cells.

The APC doesn't have to be a B-cell though. As I said, it could be a dendritic cell or a macrophage. The process is the same. Macrophages/dendritic cells phagocytose the antigen and present peptides on MHC II molecules on their surface. Helper T cells then bind to the MHC II: peptide complex, which activates the helper T cell. The activated helper T cell proliferates and differentiates into effector helper T cells and memory T cells. The effector helper T cells secrete cytokines which can activate B-cells (amongst other things), causing B-cells to proliferate and differentiate, as I mentioned previously.
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