Figure 1. Vaccine administration routes.

The main administration routes for vaccines are mucosal and parenteral. (A) Mucosal administration refers to the administration mainly through the nasal, oral or vaginal routes. In all these cases, nanocarriers need to reach the mucosal-associated lymphoid tissues (MALT). This can be principally achieved either by a paracellular or transcellular across the microfold (M) cells. At the level of M cells or underneath the epithelium, nanocarriers will encounter the resident dendritic cells and activate them, generating a mucosal immunity while, at the same time, some dendritic cells will drain to the closest lymph node and activate a systemic immune response. (B) Parenteral administration includes subcutaneous, intramuscular or intradermal injection of the nanosystems. The nanocarriers are deposited in the interstitium, where they can have two different fates: self-drain to the closest lymph node or be taken up by migratory dendritic cells, which then will migrate to the closest lymph node.