It is also the main immunoglobulin in a variety of secretions such as saliva, milk, and the mucus lining the airways and digestive tract. IgA antibodies increase in amount during mucosal infections. Secretory IgA molecules are joined in pairs and complexed with a secretory component which helps in the transport of the antibodies through cells and protects the antibodies after release should they come into contact with protein-dissolving enzymes. The IgA antibodies bind to micro-organisms such as bacteria and protect us against infection by preventing them from attaching to the epithelial surface to gain entry. Then, if the person comes into contact with the infectious agent at a later time, their immune system reacts quickly and strongly against the agent and either prevents the infection from gaining hold or limits it to a relatively mild form.
Immunity is the ability of the body to resist particular infections.
So vaccination is a process to induce immunity to particular diseases, while immunity is the ability of the body to resist infection. Vaccination is a way of reducing the occurrence of infectious diseases in populations and in some cases eliminating them entirely. For example, smallpox has been eliminated by the implementation of vaccination programmes world-wide. Vaccination can also be used to protect more vulnerable people such as babies and young children from infection. The immune system can become weakened to the point where it can no longer fight off infections that would otherwise be relatively harmless.