Immunology Basics - Immunological Computation: Theory and Applications - page 16

B cell follicular
area

GC

T cell zone

Vein

Artery

Figure 1.11 A diagram showing a section of a splenic lymph node.
(From Thorbecke, G. J. and V. K. Tsiagbe. The Biology of Germinal Centers in
Lymphoid Tissue , Springer, Berlin, 1998; Liu, Y. J., G. Grouard, O. de Bouteiller,
and J. Banchereau. Int. Rev. Cytol . , 166, 139-179, 1996.)

A GC is dynamically formed when antigen-activated B cells migrate into pri-
mary follicles of the peripheral lymphoid organs. However, the formation of GCs
requires B cell activation and migration, T- and B cell interactions, and a network of
follicular dendritic cells (FDC). A GC provides a specialized microenvironment to
perform many critical immune functions such as a B cell's somatic hypermutation,
clonal expansion, a nity maturation, and diff erentiation as memory or plasma
cells. A GC is a biological immune system's functional module, which plays a major
role in immune response (MacLennon, 1994; Todd and Spickett, 2005) .
Figure 1.11 shows various zones where specifi c B- and T cell activities take place.
h e purpose of these activities is to generate a group of B cells that has the highest
capability of recognizing a stimulating antigen (Liu et al., 1996; h orbecke and
Tsia gbe, 1998).
As shown in Figure 1.12, the number of centrocytes increases in the GC, two
distinct regions begin to be distinguished:
1. h e dark zone, where proliferation centroblasts are packed closely together
and where there are few follicular dendritic cells, it is formed in a few days
within a primary lymphoid follicle.
2. h e light zone, in which centroblasts give rise to centrocytes that enter the
follicular dendritic cells network, thereby, densely packed centrocytes make
contact with the numerous cells of the follicular dendritic cell network.

Helper T cells that migrate to the primary follicle along with the activated B cells
also undergo some clonal expansion and can be seen intermingled with centrocytes
in the light zone. Centrocytes that fail to take up antigen from follicular dendtritic
cells, die and are phagocytosed by local macrophages; also, cell death in the GC is