Age-dependent changes in the anatomical and histological characteristics of the aggregated lymphoid nodules in the stomach of Dromedary camels (Camelus Dromedarius)

**Background:** Gastrointestinal-associated lymphoid tissue (GALT) is the largest lymphoid tissue mass in the body, comprising over 70% of immune cells. Peyer's patches (PPs) are key antigen-inductive sites in the intestinal mucosa. While organized lymphoid nodules are well-documented in the intestines and appendix, their presence in the stomach has rarely been described. Dromedary camels (Camelus dromedarius) have unique anatomical adaptations to desert environments, including a three-compartment stomach. This study investigates a previously unreported area of well-developed aggregated lymphoid nodules in the abomasum of Dromedary camels across different ages.

**Methods:** Twenty healthy Dromedary camels from a slaughterhouse were divided into four age groups (n=5 each): young (0.5–2 years), pubertal (3–5 years), middle-aged (6–16 years), and old (17–20 years). The abomasum was examined anatomically, with measurements of mucosal fold height and width taken using a digital vernier caliper. Tissue samples from the dorsal wall, reticular mucosal folds (RMF), and longitudinal mucosal folds (LMF) of the initial part of the abomasum were processed for histology (H&E staining, 4μm paraffin sections) and immunohistochemistry (SABC method using rabbit polyclonal antibodies against Bactrian camel IgG, with DAB visualization). Statistical analysis used one-way ANOVA with Duncan's multiple range test (IBM SPSS V.23.0), with significance set at p < 0.05.

**Key Results:** The aggregated lymphoid nodule area (ALNA) was located on the ventral wall of the initial enlarged part of the abomasum along the minor curvature, forming a triangular zone between the gastric vessels and the left greater omentum attachment. It consisted of two regions: a reticular region with grid-like membranous folds (thickness 5–6 cm) and a linear region with longitudinal membranous folds extending 16–18 cm along the minor curvature. The mucosal folds in ALNA were significantly thicker and more elevated than surrounding mucosa. Histologically, abundant lymphoid follicles were found in the submucosa of both RMF and LMF, arranged in one or multiple rows. Follicles had distinct germinal centers, follicle-associated epithelium (FAE), subepithelial dome areas (SED), and high endothelial venules in interfollicular areas. Statistical analysis showed that both height and width of longitudinal and reticular mucosal folds peaked in pubertal animals and then gradually decreased with age. Significant differences (p < 0.05) in fold heights and widths were found between most age groups. Immunohistochemistry revealed that both IgA and IgG antibody-producing cells (APCs) were extensively distributed in the gastric lamina propria among the gastric glands in all age groups. IgA+ APC densities were consistently higher than IgG+ APC densities in the longitudinal area across all groups. Both APC populations increased from young to pubertal age, peaked in pubertal animals, and decreased in old age, mirroring the anatomical changes.

**Clinical Implications:** This is the first report of organized aggregated lymphoid nodules in the stomach of any animal or human, representing a significant discovery in mucosal immunology. The ALNA in the Dromedary camel abomasum is structurally and immunologically analogous to intestinal Peyer's patches, suggesting the camel stomach plays an active role in mucosal immunity beyond digestion. The age-dependent development—peaking at puberty and regressing in old age—parallels the involution pattern of lymphoid tissues seen in other species. The presence of both IgA and IgG APCs indicates that this structure participates in both mucosal and systemic humoral immune responses. These findings provide immunomorphological evidence for the camel's well-known disease resistance and open new avenues for comparative immunology research. The absence of similar structures in other species suggests this may be a unique adaptation of camels to their desert environment and pathogen exposure.