Histology of the tonsils and appendix

Features of lymphoid follicles and cellular response

A lymphatic follicle (nodule) in mucous membranes has a clear zonation. In its center lies the reactive (germinal) center, where proliferation and complex differentiation of B lymphocytes take place. This center is morphologically divided into light basal and light apical zones. Peripherally, the follicle is surrounded by a dark corona (mantle zone), which is densely populated by small lymphocytes.

In the palatine tonsils, two types of humoral response can be observed, depending on the lymphocyte population:

• $B_1$ lymphocyte response: Characterized by high speed but low specificity. This is a T-independent process—it does not require T helpers to be triggered. There are no complex genetic rearrangements (somatic hypermutation), affinity maturation, or antibody class switching. The cells synthesize predominantly IgM and do not form memory B cells.
• $B_2$ lymphocyte response: Represents a classic, complete variant of the humoral reaction. The presence of well-developed reactive centers in the follicles indicates precisely the progression of all stages of specific immunogenesis.

Microscopic structure of the appendix

The vermiform appendix is anatomically an outgrowth of the cecum. Like the palatine tonsil, its mucosa is penetrated by numerous invaginations—crypts. On histological sections, these crypts may look like separate fragments due to different cutting angles (transverse or oblique).

The epithelial lining of the appendix is fundamentally different from that of the tonsils. It features a simple columnar epithelium typical of the intestine, whereas in the tonsils it is stratified. The epithelial nuclei are located basally, and the apical part of the cells stains oxyphilic (pink).

The lymphoid apparatus is located in the lamina propria of the mucosa (in loose fibrous connective tissue) directly beneath the crypts. It encompasses the entire circumference of the appendix and is represented by both follicles and parafollicular tissue. A characteristic feature is robust lymphocytic infiltration: lymphocytes densely permeate the connective tissue above the follicle and actively penetrate directly into the epithelial layer.

Inductive phase of the immune response in mucous membranes

The initiation of an immune reaction in mucous membranes begins with the capture of foreign material. Specialized M cells capture antigens directly from the organ lumen via endocytosis and transcytosis. A crucial feature of this stage: the antigen is transported into M cell pockets in its native form, without cleavage.

Next, in the subepithelial zone (or within the M cell pockets themselves), the antigen is recognized by B lymphocytes and nonspecifically captured by macrophages. Antigen-presenting cells process the material and display peptides on their membrane. Afterward, the activated cells leave the subepithelial zone and migrate into the parafollicular (T-dependent) region.

There, cellular cooperation takes place, analogous to processes in systemic lymph nodes:

1. T helpers recognize peptides on the surface of antigen-presenting cells and begin to proliferate.
2. B lymphocytes receive stimulating signals, proliferate, and head to the reactive centers of follicles to complete maturation (somatic mutagenesis, clone selection, $C_H$ class switching).

Lymphocyte homing and IgA secretion

Having completed the initial stages of differentiation, proplasmacytes and active $T_{h2}$ helpers leave the follicle and enter the systemic circulation. Their route runs through the lymph to the regional lymph node, then to the thoracic duct, and finally into the venous bed. Interestingly, cells do not linger in lymph nodes, but strive to enter the bloodstream.

Next, the process of homing takes place—the recirculation and return of cells back to the mucous membranes. The appearance of special adhesion molecules on the endothelium of postcapillary venules serves as a stimulus for cells to settle in the lamina propria of the mucosa (this can be either the original focus or another inflamed mucosa).

At the final (productive) stage, proplasmacytes finally turn into mature plasmacytes and begin to actively secrete immunoglobulin A (IgA). Epithelial cells capture this IgA and transport it via transcytosis to the mucosal surface. During passage through the endosome of the epithelial cell, a secretory component (S peptide chain) attaches to the antibody. It is this component that protects the molecule from the destructive action of digestive enzymes in the organ lumen.

Briefly: answers to topical questions

Class and exam questions

What else to study on this topic

• Mechanisms of somatic mutagenesis and selection in germinal centers
• Structure and functions of mucosal M cells
• The process of isotypic antibody class switching ($C_H$ switching)
• Molecular bases of homing: the role of adhesion molecules on venules
• Interaction of macrophages and B lymphocytes in the subepithelial zone

Other topics in the section "Hematopoietic and lymphoid organs"

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