Again, germinal centers can be produced
in response to antigenic stimulation.
And really the germinal center can be thought of
as the power house of the adaptive immune response.
It’s really where all the cells that are necessary
to generate an adaptive immune response get together.
So you’ll have B-cells, you’ll
have T-cells, you’ll have dendritic
cells, you’ll have follicular dendritic cells and so forth.
And although there are these distinct zones as we’ve
seen, T-cell areas and B-cell areas in the secondary
lymphoid tissues, of course the B-cells and the
T-cells are not entirely separated from each other.
They actually need to get together because some
of the cell interactions that are required,
require molecules on the surface of T-cells to
interact with molecules on the surface of B-cells.
So clearly the cells have to
come in to cell-cell contact.
They can’t be entirely
separated from each other.
So here we see in the top left, a tissue
section that is stained to clearly
demarcate the outer region of the
germinal center, the mantle zone.
And within the germinal center
itself, two zones that stain with
different intensities - a light zone and a darker staining zone.
And these are characteristic
of the germinal center.
Looking now at an immunofluorescent
section, again we can see the outer mantle
zone and then within the germinal center
itself, the light zone and the dark zone.
Looking at B-cell migration into the germinal
center, B-cells within the rest of the secondary
lymphoid tissue will get together with helper T-cells
and generate these germinal center structures.
What happens in the dark zone of the germinal center is that
there is extensive proliferation, cell division of the B-cells.
Within the light zone, there is somatic
hypermutation, there is affinity
maturation and there is isotype switching
or class switching of the B-cells.
And this requires interaction of the B-cells with
so called follicular helper T-cells, TFH, which
is a population of helper T-cells that are specialized
to help the B-cells in the germinal center.
Follicular dendritic cells will show antigen to
the B-cell receptor on the surface of the B-cell.
So the end result, is that there will be
activation and fine tuning of the B-cell response,
there will be differentiation of the B-cells
into both memory B-cells and into plasma cells.
And there will be the exit of high affinity antibody secreting
B-cells and memory B-cells from the germinal center.