7. The Secondary Lymphoid Nodule (Fig. 3. below) Lymphoid nodules
start out as simple spherical accumulation of lymphocytes in response
to antigenic stimulation. With time germinal centers form in the center
of these lymphocyte spheres. At that time these nodules are classified
as secondary lymphoid nodules. A simple phrase for identification would
be a "lymphoid nodule with germinal center." Secondary lymphoid
nodules develop toward the end of the primary immune response as antibodies
must be present in the system to trap antigens by follicular dendritic
cells (an antigen presenting cell) for the stimulation of B cells. See
cell types below:
a. Light region of germinal centers. The light region is known
to contain the following cells:
1. Some reticular cells of the stroma;
2. Several hundred Follicular Dendritic Cells (FDC) ---
function as antigen repository. These cells can trap immune complexes
via the Fc portion of the antibody (Ab) in the immune complex. Immune
complexes bind complement molecules (C')and these help bind the immune
complexes (Ab-Ag-C') to C'-receptors on the FDCs. With time FDCs produce
small immune complex coated spheres (~3mm
in diameter).
3. Mostly small B lymphocytes --- A few with
antigen specific surface receptors (abbreviated as BCR). B-cells
with antigen specific receptors bind to the FDC-immune complexes for
antigeninc stimulation. These B cells endocytose iccosomes and process
the antigen taken in with the iccosome. This processed antigen is
re-expressed on the B cell surface for presentation to "processed-antigen
specific" T helper cells.
4. Very few T helper cells --- These interact,
in the microenvironment provided by the dendrites of the FDCs, with
the B cells and stimulate them to mature into memory B cells and antibody
producing plasma cells.
b. Dark region of germinal centers --- Contains mostly
activated B cells with lots of ribosomes in their cytoplasm. The basophilia
of these ribosomes gives the darker basophilic staining to this region
of the germinal center. The cells known to be present in this region
are primarily:
1. Few reticular cells of the stroma.
2. Very few small B-cells. --- May represent
maturing B memory cells.
3. A few immunoblasts. --- These are stimulated B cells that
form blasts and will proliferate to replicate the specific B cells.
4. Mitotic figures --- Intermediate stages of cell division
of Immunoblasts.
5. Mostly medium size B cells --- derived from the immunoblasts
and will mature into small B memory cells or plasma cells which eventually
migrate to the bone marrow to produce antibodies.
6. An occasional plasma cell. --- A fast maturing cell.
7. Tingible Body Macrophages --- Identifiable by the content
of darkly stained remnants of nuclei ("tingible bodies")
of phagocytized apoptotic B lymphocytes