Lymphoid Follicles and Cellular Response
Lymphoid follicles (nodules) in mucous membranes have a distinct zonation. In the center lies the germinal center (reactive center), where proliferation and complex differentiation of B lymphocytes occur. Morphologically, this center is divided into light basal and light apical zones. Peripherally, the follicle is surrounded by a dark mantle zone (corona), which is densely populated by small lymphocytes.
In the palatine tonsils, two types of humoral responses depend on the lymphocyte populations:
- $B_1$-Lymphocyte Response: Characterized by high speed but low specificity. This is a T-independent process—it does not require T-helper cells for activation. It lacks complex genetic rearrangements (somatic hypermutation), affinity maturation, and class switching. These cells predominantly synthesize IgM and do not form memory B cells.
- $B_2$-Lymphocyte Response: Represents the classical, complete humoral response. Well-developed germinal centers in the follicles indicate the active progression of specific immunogenesis.
Microscopic Structure of the Appendix
The vermiform appendix is anatomically an outpocketing of the cecum. Like the palatine tonsil, its mucosa is penetrated by numerous depressions called crypts. In histological sections, these crypts may appear as isolated fragments due to various cutting angles (transverse or oblique).
The epithelial lining of the appendix differs fundamentally from the tonsils. It features a simple columnar epithelium typical of the intestine, whereas the tonsil epithelium is stratified. Epithelial nuclei are located basally, and the apical portion of the cells stains oxyphilic (pink).
The lymphoid tissue is located in the lamina propria (within loose fibrous connective tissue) directly beneath the crypts. It encircles the entire lumen of the appendix and is represented by both follicles and interfollicular tissue. A characteristic feature is robust lymphocytic infiltration: lymphocytes densely permeate the connective tissue above the follicle and actively migrate directly into the epithelial layer.
Inductive Phase of the Mucosal Immune Response
The initiation of an immune reaction in mucosal membranes begins with the capture of foreign material. Specialized M cells (microfold cells) capture antigens directly from the organ lumen via endocytosis and transcytosis. A crucial feature of this stage is that the antigen is transported into the M-cell pockets in its native form, without degradation.
Next, in the subepithelial zone (or within the M-cell pockets), antigens are recognized by B lymphocytes and nonspecifically engulfed by macrophages. Antigen-presenting cells process the material and display peptides on their membranes. Afterward, the activated cells leave the subepithelial zone and migrate into the interfollicular (T-dependent) area.
Cellular cooperation occurs there, analogous to processes in systemic lymph nodes:
- T-helper cells recognize peptides on the surface of antigen-presenting cells and begin to proliferate.
- B lymphocytes receive stimulating signals, proliferate, and migrate into the germinal 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, plasmablasts and active $T_{h2}$ helpers leave the follicle and enter systemic circulation. Their route runs via lymph to regional lymph nodes, then into the thoracic duct, and finally into the venous system. Interestingly, cells do not linger in the lymph nodes but instead head toward the bloodstream.
Next, homing occurs—the recirculation and return of cells back to the mucous membranes. The appearance of specialized adhesion molecules on postcapillary venule endothelium serves as a stimulus for cells to settle in the lamina propria (either the original site or another inflamed mucosa).
In the final (productive) stage, plasmablasts fully transform into mature plasma cells and begin actively secreting immunoglobulin A (IgA). Epithelial cells capture this IgA and transport it via transcytosis onto the mucosal surface. During transit through the epithelial cell endosomes, a secretory component (peptide chain S) is attached to the antibody. This component protects the molecule from the destructive action of digestive enzymes in the lumen.