Germinal Center – Lymphocyte Activation

00:01 Again, germinal centers can be produced in response to antigenic stimulation.

00:07 And really the germinal center can be thought of as the power house of the adaptive immune response.

00:14 It’s really where all the cells that are necessary to generate an adaptive immune response get together.

00:19 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.

00:39 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.

00:50 So clearly the cells have to come in to cell-cell contact.

00:52 They can’t be entirely separated from each other.

00:55 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.

01:07 And within the germinal center itself, two zones that stain with different intensities - a light zone and a darker staining zone.

01:16 And these are characteristic of the germinal center.

01:21 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.

01:33 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.

01:49 What happens in the dark zone of the germinal center is that there is extensive proliferation, cell division of the B-cells.

01:58 Within the light zone, there is somatic hypermutation, there is affinity maturation and there is isotype switching or class switching of the B-cells.

02:10 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.

02:27 Follicular dendritic cells will show antigen to the B-cell receptor on the surface of the B-cell.

02:36 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.

02:51 And there will be the exit of high affinity antibody secreting B-cells and memory B-cells from the germinal center.