How is a B-cell activated?

Was doing a Biology past paper and for the question (in the title) I put:
1- Bcell with complementary receptor on cell membrane binds to antigen
2- T-helper cell is activated by binding to antigen presenting cell (APC) and therefore many active T-helper cell clones produced which then produce cytokines
3- Cytokines stimulate proliferation of active Bcell clones and then subsequent differentiation into B-effector and B-memory cells.

But Mark scheme says:
1{antigen / bacteria / virus / pathogen} {binds / eq} to B cell ;
2. {antigen / bacteria / virus / pathogen} {binds / eq} to MHC (antigen) 3. T helper {lymphocytes / cells} {bind / eq} (to B cell) ;
4. reference to cytokines (from T helper cells) ;

Is the mark scheme referring to the other way of activation of Bcells (Bcell becomes an APC then activated by Bcell binding to T-helper cell)??

So for a B cell to become activated, it must encounter its cognate antigen (i.e. the antigen to which the B cell receptor is complementary to) and it must also receive help from a T helper cell. T helper cells become activated when they recognise and bind to their cognate their antigens (again, the antigen to which the T cell receptor is complementary to) which are presenting on MHC molecules on the surface of antigen presenting cells (e.g. dendritic cell or macrophage). Once the T cell and B cell have recognised their antigens, the T cell binds to the B cell (the B cell also presents antigens on MHC molecules on their surface, they are professional antigen presenting cells!) and surface molecules on the T cell binds to receptors on the B cell and also secrete cytokines which activate the B cell, causing it to proliferate and differentiate into plasma cells and memory B cells.

ah okay, its because in the SNAB book it said B-cell becomes APC then binds to T-helper and cytokines released, then proliferation of B-cells and differentiation.
But my teacher said that there are different ways of becoming activated and the way he said was the one I put down in my answer in OP... so for all methods of B-cell activation, it HAS to BIND to T-helper cell?

Yes, a T-cell has to bind to a B cell to activate it - the T cell binds to the antigen presented on MHC molecules on the surface of the B cell using its T cell receptor and also expresses costimulatory molecules (and also secretes cytokines) which activate the B cell.

For some types of antigens (e.g. endotoxin/lipopolysaccharide on gram negative bacteria), B cells can become activated without help from T cells - however, at your level I wouldn't worry about this.

Are T-memory cells created when T-helper cells are activated?

Also, are T-killer cells activated when they bind to pathogen-infected host cell (which therefore presents pathogen's antigen) and then cytokines produced by T-helper cells cause proliferation of T-killer cells?

and in your first reply when you said t-cells bind to b-cells, is it the CD4 receptor on t-helper cells that bind to b-cells MHC complex to activate the b-cell?

Yes, when a T helper cell is activated, it proliferates and differentiates into memory T helper cells and effector T helper cells - which release cytokines, activate/recruit phagocytes, etc.

The precise mechanisms by which cytotoxic T cells are activated are a bit complex. Cytotoxic T cells are activated when they bind to antigen presented on MHC I molecules on the surface of antigen presenting cells AND receive cytokines release by T helper cells. Once they are activated they will proliferate and differentiate into memory cytotoxic T cells and effector cytotoxic T cells (i.e. the ones that actually do the killing).

It’s the T cell receptor that recognises and binds to the antigen presented on the MHC II molecule, the CD4 is a coreceptor - it binds to specific regions on the MHC II molecule itself, rather than the antigen. T helper cells have CD4, cytotoxic T cells have CD8. For a B cell to become activated, the T cell binds to the antigen-MHC complex using its T cell receptor, CD4 binds to the MHC II molecule, and a protein on the surface of the T helper cell (CD40L) binds to a receptor on the B cell (CD40) - plus the T helper cell secretes cytokines.

Immunology can be very challenging!

Thanks v much! So for biology A-level (Salters nuffield) is the following correct:

B-cell activation:
B-cell binds to antigen and presents it with MHC protein on cell surface membrane as an antigen-MHC complex
T-helper cell with complementary receptor binds to antigen-MHC complex on B-cell membrane.
Cytokines released by T-helper cells which stimulate proliferation (clonal expansion) of active B-cell clones and then subsequent differentiation of active B-cells into B-effector cells and B-memory cells

T-helper cell activation:
Phagocyte (macrophage/neutrophil) phagocytoses pathogen (it is engulfed in a phagocytic vesicle which then fuses with lysosomes containing lysozyme enzyme which then digests pathogen)
Antigens bind with MHC protein and thus antigen-MHC complex presented on cell surface membrane of phagocyte
Phagocyte is now an Antigen Presenting Cell (APC)
T-helper cell with complementary CD4 receptor binds to antigen-MHC complex on Phagocyte
T-helper cell proliferation stimulated and thus clones of active T-helper cells produced and some T-memory cells produced

T-killer cell activation:
T-killer cell with complementary receptor binds to a pathogen-infected host cell (this cell is presenting the pathogen's antigens on its cell surface membrane)
Cytokines released by T-helper cells stimulate proliferation of active T-killer cells (clonal expansion)