The effect of microparticle size on gastrointestinal (GI) tissue uptake was investigated using biodegradable polylactic polyglycolic acid (PLGA) microparticles (100 nm, 500 nm, 1 µm, and 10 µm) and bovine serum albumin (BSA) as a model antigen. The study used an in situ rat intestinal loop model to evaluate uptake efficiency. Results showed that 100 nm particles had 15–250 times higher uptake efficiency compared to larger particles. Uptake efficiency varied by tissue type (Peyer's patch vs. non-patch) and location (duodenum vs. ileum). Peyer's patch tissue showed 2–200 times higher uptake than non-patch tissue. Histological analysis revealed that 100 nm particles were diffused throughout submucosal layers, while larger particles were localized in the epithelial lining. These findings suggest that smaller microparticles are more efficiently taken up by GI tissue, which has implications for designing nanoparticle-based oral drug delivery systems, such as oral vaccines. The study aimed to formulate and characterize microparticles of various sizes, evaluate their uptake in the in situ intestinal loop model, and study their histological localization. Microparticles were formulated using a water-in-oil-in-water emulsion solvent evaporation technique. Characterization included particle size distribution, BSA loading, SDS-PAGE, and in vitro release studies. The results indicate that microparticle size significantly affects GI tissue uptake, with smaller particles showing greater efficiency. This has important implications for the development of oral drug delivery systems.The effect of microparticle size on gastrointestinal (GI) tissue uptake was investigated using biodegradable polylactic polyglycolic acid (PLGA) microparticles (100 nm, 500 nm, 1 µm, and 10 µm) and bovine serum albumin (BSA) as a model antigen. The study used an in situ rat intestinal loop model to evaluate uptake efficiency. Results showed that 100 nm particles had 15–250 times higher uptake efficiency compared to larger particles. Uptake efficiency varied by tissue type (Peyer's patch vs. non-patch) and location (duodenum vs. ileum). Peyer's patch tissue showed 2–200 times higher uptake than non-patch tissue. Histological analysis revealed that 100 nm particles were diffused throughout submucosal layers, while larger particles were localized in the epithelial lining. These findings suggest that smaller microparticles are more efficiently taken up by GI tissue, which has implications for designing nanoparticle-based oral drug delivery systems, such as oral vaccines. The study aimed to formulate and characterize microparticles of various sizes, evaluate their uptake in the in situ intestinal loop model, and study their histological localization. Microparticles were formulated using a water-in-oil-in-water emulsion solvent evaporation technique. Characterization included particle size distribution, BSA loading, SDS-PAGE, and in vitro release studies. The results indicate that microparticle size significantly affects GI tissue uptake, with smaller particles showing greater efficiency. This has important implications for the development of oral drug delivery systems.