Germinal Center

Histologically , the GCs describe microscopically distinguishable parts in Lymphoid tissues.

1. Activated B-cells migrate into the Follicular System and begin Monoclonal expansion in the environment of Follicular Dendritic Cell s (FDC).

2. After three days of expansion the B cells mutate their
Antibody - Encoding DNA and thus generate a diversity of Clone s in the
germinal centre. This also involves deletions, insertions and
Recombination of the V, D, J genes.

3. Upon some unidentified stimulus
from the FDC, the B cells start to expose their antibody to their
surface and in this stage are referred to as Centrocyte s. The
centrocytes are in a state of Activated Apoptosis and compete for
survival signals from FDCs that present the
antigen. This rescue process is believed to be dependent on the Affinity
of the antibody to the antigen.

4. The functional B-cells have then to interact with helper T cells to get final Differentiation Signal s. This also involves Isotype Switching for example from IgM to IgG .
The interaction with T cells is believed to
prevent the generation of autoreactive antibodies.

5. The B cells become either a Plasma Cell
Spread ing antibodies or a Memory Cell that will be activated in
subsequent contacts with the same antigen. They may also restart the
whole process of Proliferation , Mutation and Selection according to the
Recycling hypothesis.

The Morphology of GCs are very
Specific and shows properties which are Characteristic for different
Stages of the reaction. In an early state of the reaction a network of
FDCs is fully filled with proliferating B cells. Later at day 4 of the
reaction GCs show a separation of two zones, the dark and the light
zone. The former still contains dominantly proliferating cells while the
latter one is the area of B cells selection. These zones
dissolve after 10 days of GC development which ends after about 3 weeks.

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Germinal Center

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