Just a recap...
Check out Lymph node 101 post for a recap of basic LN anatomy.
Upon entering the LN, B cells "home" to the superficial cortex and form follicles. If the B cells come in contact with a "bad guy" (called antigenic stimulation), they will become activated and undergo systematic transformational changes as they mature to become plasma cells.
Cortex- Follicles & Germinal centers= B cells
Paracortex= T cells
Medulla- cords, sinuses, trabecula= Plasma cells
Reminder: T cells help to activate B-cells, thus they tend to be located surrounding the B-cell follicles, but are deeper in the cortex (paracortical region).
The anatomy of a follicle
In a "normal" LN, follicles should be:
Located around the periphery
Show progressive maturation. As the B-cells mature and are activated (due to infection or another cause), the follicles will change from a quiescent (resting) primary follicle to secondary follicles with germinal centers.
Plasma cells function to produce antibodies against the antigen (think of it as a way to mark all of the "bad guys" so the body knows what to attack). Plasma cells will move from the follicles in the cortex to deeper within the LN in the medulla where they line up along the medullary sinusoids. They then release their antibodies into the efferent lymphatic fluid so it can go to other LNs around the body to alert them that there is a "bad guy" around that needs to be taken care of.
Secondary follicles (Germinal Centers)
Germinal centers form upon B cell activation as the B cells are exposed to some pathogen, then proliferate and eventually mature to plasma cells. The germinal centers consist of centroblasts (large, noncleaved cells with moderate amounts of basophilic cytoplasm, round nuclei, open chromatin and multiple peripheral nucleoli) and centrocytes (small cleaved cells with scant cytoplasm).
During the follicular B-cell activation process, a dark zone (containing the proliferating B cells) and a light zone (where the B cells further specialize into plasma cells) can be appreciated. Surrounding the periphery of the germinal center is what is referred to as the Mantle Zone. The mantle zone contains small unchallenged B cells.
Surrounding the mantle zone is another light staining zone referred to as the Marginal Zone. The marginal zone contains postfollicular memory B cells that are derived after stimulation of recirculating cells from T cell dependent antigens. Marginal zones are mostly seen in the spleen at the interface of lymphoid (white) and red pulp and rarely seen in lymph nodes (except mesenteric LNs).
T cells and dendritic cells (macrophages) are located deeper within the cortex in the paracortical zone. The medulla contains medullary cords (activated B cells, plasma cells) and medullary sinuses (lymph, more macrophages, plasma cells and reticular cells).