Morphology of lymphoid follicles

Germinal center: light and apical zones

Reactive (germinal) centers of follicles represent the B-zone of active antigen-dependent proliferation. Several functional areas are distinguished here, where B lymphocytes undergo sequential stages of development.

In the light zone, a unique "exam" takes place for centrocytes (B cells after mutagenesis). This process is called positive selection. Dendritic cells present uncleaved antigens in complex with IgM to centrocytes. The key survival criterion is high affinity of the cell receptor for the antigen. Those cells whose affinity turns out to be low (which is about 90% of the population) undergo apoptosis, after which they are phagocytosed by large macrophages. Surviving centrocytes either return to the dark zone for repeated mutations or proceed to the next stage.

In the light apical zone, selected B cells transform into large, actively dividing B immunoblasts (plasmablasts) with a pale nucleus and cytoplasm. It is here that the critical genetic rearrangement occurs — $C_H$ class switch recombination. The cell stops synthesizing primary antibodies (IgM) and starts producing other classes: most commonly IgG, less often IgA or IgE.

Follicular corona (mantle)

After division in the apical zone, daughter cells move to the periphery of the follicle — the mantle zone (corona). In this B-zone, their paths finally diverge into two directions:

1. Formation of proplasmocytes. Some cells begin terminal differentiation to eventually become effector cells of humoral immunity — plasmocytes.
2. Formation of memory B cells. These cells differ from naive lymphocytes by bearing already-switched immunoglobulin classes (IgG, IgA) on their membrane. Externally, they closely resemble small lymphocytes: they have a hyperchromatic nucleus and a narrow rim of cytoplasm.

Memory B cells play a crucial role in the secondary immune response. Upon re-exposure to the same antigen, the body's reaction is faster and more powerful due to a larger number of ready cells, their high affinity, and amplification of immunoglobulin genes.

Tissues outside the follicle

The life cycle of lymphocytes is not limited to follicles. Development continues in deeper structures of the lymph node.

The paracortical zone is thymus-dependent (T-zone) and located between the follicles and the medulla. It houses T lymphocytes (helpers and killers) during stages of antigen-dependent differentiation. The main antigen-presenting cells of this zone are interdigitating cells, which present peptide antigens to T lymphocytes. Some B cells may pass through the paracortex in transit, entering the bloodstream directly.

Medullary cords are a B-zone located deep within the node. Proplasmocytes "flow" here from the corona for final maturation, as this process never occurs inside the follicle. The tissue of the cords contains mature plasmocytes, proplasmocytes, and typical macrophages.

Morphology and identification of plasmocytes

Plasma cells are the final stage of B lymphocyte development, true cellular "factories" for antibody synthesis. These are large cells with an eccentrically located nucleus dominated by active euchromatin.

Plasmocytes require a powerful apparatus to synthesize protein molecules. Their cytoplasm is packed with rough endoplasmic reticulum (rER), and a well-developed Golgi apparatus is responsible for antibody packaging. Under light microscopy, the Golgi zone appears as a "hof" (pale area) next to the nucleus.

Histochemically, plasmocytes are easily identified by RNA staining, for example, with pyronin. Due to the huge number of ribosomes on the cytoplasmic reticulum, the cell accumulates a lot of RNA, giving its cytoplasm a specific crimson color (pyroninophilia). Such stained cells are localized predominantly in the medullary cords of the lymph node. Having fulfilled their function, they may remain there or leave the node via medullary sinuses and high endothelial venules.

Briefly: answers to topical questions

Class and exam questions

What else to study on this topic

• Structural features and functions of the dark zone of the germinal center.
• Mechanism of immune complex presentation by dendritic cells.
• Molecular processes during genetic rearrangement ($C_H$ class switch recombination).
• Interaction of T helper cells and B cells in germinal centers.
• The role of interdigitating cells in the paracortical zone.

Other topics in the section "Hematopoietic and lymphoid organs"

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