**7. Conclusions**

*Chitin and Chitosan - Physicochemical Properties and Industrial Applications*

administered in the body and recognised by the immune system.

to protect the body against the pathogen and their toxin [54].

surface and triggers the activation of naive CD4+

**5. Challenges in oral vaccine delivery system**

Vaccination is one of the most cost-effective approaches to prevent infectious diseases such as hepatitis B, tetanus, polio, and rabies. Vaccines contain pathogens, either live-attenuated, inactive or killed antigen [75]. These pathogens will be

The oral delivery of vaccines is quite challenging as the pathogen is introduced into the body. It is mandatory to ensure mucosal immune response works effectively

As the vaccine enters the intestine, its presence will trigger the inductive site, the Peyer's patches. The Peyer's patches consist of M-cell which will allow the entry of the antigen through endocytosis. The antigen then will be transported into intraepithelial dendritic cells or macrophages and be taken up by the cell through

The antigen-loaded dendritic cell will present the antigen fragment on its

will bind to the antigen fragment, MHC class II. This binding releases chemical mediators, interleukin-2 (IL-2), that function to regulate the activity of lymphocytes for immunity. IL-2 stimulates the cell division of CD4+ T-cells, activates B-cells and cytotoxic T-cells. B-cell is responsible for mediating humoral immunity by differentiating into plasma cells. Plasma cells will generate antibodies to fight

Viral protein requires the right structural conformation to attach to the host cell and replicate. Highly acidic in the stomach and extreme temperature changes will cause protein denaturation. The denaturation of the virus will alter the conformation of its structure [58]. The high temperature will break the phosphodiester bond. However, at low temperature, the degradation of the nucleic acid will also lead to

Furthermore, to transport vaccines orally, it should be able to overcome the biological barrier of the intestinal epithelial cell such as tight junction and mucus. The hydrophilic antigen cannot cross the phospholipid bilayer to enter systemic circulation due to the function of tight junction in controlling the permeability of the membranes. Therefore, the uptake of the antigen to mucosal tissue is limited

The GIT contains normal flora or microbiota which help in maintaining the structure of the gut mucosal barrier [55]. Those microbiotas not only aid nutrient metabolism, but they also possess an action to protect against invading pathogens [79, 80]. Well-balance microbiota is needed to induce the effectiveness of vaccines through oral administration. The delivery of the vaccine will be interrupted in patients with microbiota dysbiosis, leading to blunted vaccine

The induction of danger signals appropriately by the vaccine is essential to trigger an immune response [81]. Due to these limitations, there is a problem in inducing an adequate immune response against administered pathogens [3]. Consequently, a higher and repetitive dose is required. Nevertheless, the administration of high antigen doses repetitively may develop systemic oral tolerance

T-cells. The activated CD4+

T-cells

**4. Oral vaccine delivery system**

phagocytosis [76].

against pathogens [77].

viral inactivation [78].

response [80].

towards vaccines [3, 78].

with a short time of exposure [78].

**64**

Chitosan-based drug formulation has gained attention for their ability to serve as a carrier and an enhancer for oral delivery of peptides and vaccines. Although oral delivery is the most convenient and preferred route of administration, however, it has limitations due to the presence of the proteolytic enzyme, pH of GIT and the intestinal barrier to drug absorption. In recent years, there has been considerable research interest in the application of chitosan as an enzyme inhibitor, mucoadhesive agent and efflux pump inhibitor. Interaction of positively-charged amino groups of chitosan with negatively-charged sialic acid groups that exist in mucin prolongs the residence time between drugs and membranes, therefore enhancing the bioavailability of the drugs. Other formulation strategies include encapsulation of proteins, peptides and vaccines into a nanoparticulate delivery system. By encapsulating peptide into a nanocarrier system, the enzymolysis and peptides aggregation can be avoided thus enhances the absorption of peptide drugs in the intestinal epithelium. Similarly, encapsulation of pathogens in nanoparticles is a good approach to improve the effectiveness of vaccines in stimulating the immune system.

## **Acknowledgements**

The authors acknowledge Faculty of Pharmacy, Universiti Teknologi MARA and internal grant (DUCS) 2.0, 600-UiTMSEL (PI. 5/4) (016/2020).
