The term T-dependant antigen is used to denote the fact that
most B-cell responses require assistance from helper T-cells.
So an antigen will be taken up by a dendritic cell, it’ll
be processed and presented to a naïve T helper cell.
This T helper cell will then interact with B lymphocytes and
cause their proliferation and differentiation into plasma cells.
These plasma cells will go on and secrete a
soluble version of the antibody molecule.
The interaction of the helper T-cell with the
B-cell can be facilitated by the fact that the
B-cell itself can act as a professional antigen
presenting cell for antigen experienced T helpers.
In other words, dendritic cells are
required to activate naïve T-cells,
but once they have been activated
by a dendritic cell, the B-cell can
take over that function of acting as
a professional antigen presenting
cell; showing peptides to the T-cell
receptor on the helper T-cell.
However some antigens are T-independent
antigens with respect to the B-cell response.
These are antigens that tend to have repeating
structures which extensively cross-link the
B-cell receptor or cross-link Pattern Recognition
Receptors and the B-cell receptor together.
But because T-cells do not participate
in the response to these T-independent
antigens, there is no class switching and only IgM is produced.
Because in order to get class switching to
IgG or IgA or IgE production, you need CD40
ligand on the surface of the T-cell to interact
with CD40 on the surface of the B-cell.
And if there’s no T-cell involved, then
that class switching will not occur.
Examples of T-independent antigens
include bacterial lipopolysaccharide,
capsular polysaccharides of bacteria and polymeric proteins.
So here we can see a T-independent antigen
with multiple repeating structures on its
surface, powerfully cross-linking the B-cell
receptor on the surface of the B-lymphocyte.
This will cause the B-cell to proliferate
without any assistance from helper T-cells.
And those B-cells can
differentiate into plasma cells.
But those plasma cells will only secrete the IgM class
of antibody and tiny little bits of IgD antibody as well.
But they will not class switch
to produce IgG, IgA or IgE.
For T-dependant antigens which is the
vast majority of antigens, B-cells can be
induced to class switch following interaction
with appropriate helper T-cells.
So here you can see a naïve B-cell co-expressing on its cell
surface, IgM and IgD antibodies
of identical antigen specificity.
You can also see on the surface of the B-cell,
the MHC Class II peptide and the molecule CD40.
In the presence of helper T-cells, the
CD40 ligand molecule on the surface
of the T-cell interacts with the
CD40 on the surface of the B-cell.
This interaction will cause the helper T-cell to
release cytokines and this combination of CD40
ligand binding to CD40 on the B-cell together with
cytokines will cause the B-cell to class switch.
In this particular example, it is switched
to produce the IgG class of antibody.
These B-cells will then differentiate into plasma cells
and secrete a soluble version of the IgG antibody.