**6. Strategies to improve oral vaccine delivery system using chitosan**

The uptake of antigen by immune cells depends on the particle size of the antigen. The smaller size of pathogens is readily taken up by dendritic cells [75]. The same goes for peptide, encapsulation of pathogens in nanoparticles is a good approach to improve the effectiveness of vaccines in stimulating the immune system [75].

Small particle size is required to penetrate the mucus. The formulation will be excluded out from the layer of mucus if particle size greater than normal mucus pore size (100–500 nm). This leads to the interruption in the bioavailability of antigen to targeted antigen-presenting cell (APC). Therefore, the development of nanoparticulate systems is required to provide a smaller size (20 – 40 nm) of formulation [82].

Medium molecular weight chitosan (MMWC) with the degree of deacetylation of 85% has been shown to improve the delivery of ovalbumin antigen with the presence of alginate and calcium phosphate (CaP). CaP has adjuvant properties by activating the surface expression of B-cell. CaP can be coated with mucosal penetrating polymers, such as chitosan and alginate to avoid biodegradation by enzymes present in the GIT [82]. In the stomach, the alginate-chitosan-coated CaP nanoparticle delays the release of ovalbumin antigen. The antigen then will be released in the intestine and colon with a sustained-release mechanism. This nanocarrier has successfully encapsulated ovalbumin antigen with small size (< 50 nm) [82].

The antigen should be transported to the intestine and directly to the M-cell of the Peyer's patches [6]. Chitosan develops well-protected mucoadhesion by prolonging the residence time at mucosal surfaces. The uptake of antigen by epithelial cells of the intestine will be improved by chitosan. An increase in the activity of macrophages will improve the secretion of mucosal IgA and IgG [76].
