Welcome to the second part of this talk on lymphoid architecture,
we'll be looking ahead to the way in which germinal centres form within lymph nodes.
Those are the most dramatic places where lymphocyte clones
expand and get selected for refinement in the immune system, in a Darwinian selection process.
We'll then go on to consider
the clones and some more recent studies on clonal competition,
which ones win out and which ones don't.
Right at the end we'll think a little bit about how these fascinating structures of lymph nodes ever come about in the first place.
What is the ontogeny, what's the developmental process that leads to a lymph node structure?
Let's now look the way in which the immune response gets going, and leads to the formation of germinal centres.
Germinal centres are T-cell-dependent, they need help from CD4-positive T helper cells.
I'm going to show you the way in which antigens that have
arrived in a lymph node are carried on dendritic cells, and
presented to these T-helper cells to help the germinal centres develop and form.
In this video image that I'm about to show (which was made about 15 years ago),
the colour-coding shows:.
dendritic cells in a purple colour;
the high endothelial venules are the rings here in a pinker colour;
the antigen-specific B-cells (the technique for identifying those by introducing them from outside) are in yellow;
the antigen-specific T-cells are in green;
and the B-cells (highlighting the follicles) are all around here, of all specificities, in the follicular region.
Let's run the video, and follow the fate of the dendritic cells as they meet their helper T-cells.
You can see they're quite active.
This is speeded up about 2,000 fold, but still we're now in the paracortical regions,
seeing dendritic cells with their processes wandering around, and occasionally bumping into a T-cell (in green).
It does look accidental, it doesn't look as though the dendritic cells and
T-cells come together because they're attracted to each other,
they just accidentally interact.
If they do meet each other,
then sometimes they take an interest for a more prolonged period.
You can see one or two instances, here is one with the green T-cell and
a dendritic cell hanging on for a good half-hour or so,
so that's probably been stimulated into action.
This is therefore the region where it's taking place, in the paracortical areas.