How do vaccinations work?

Vaccinations contain a dead or inactive form of a pathogen, which is a disease causing microorganism (such as bacteria and viruses). When people are vaccinated, the inactivated pathogen goes through the blood stream and is exposed to white blood cells in the circulation. All cells have antigens which are protein surface markers that allow white blood cells to recognise them as being host body cells. When white blood cells called lymphocytes encounter the pathogen, they are activated because they do not recognise the foreign antigen. They then produce antibodies which have a complementary shape to the pathogen antigen so the antibodies can grab the pathogenic cells and stick them together in a clump. Another white blood cell called phagocyte then engulfs the clump of pathogen and antibodies and destroys it in a process called phagocytosis. When all the pathogens are destroyed, a small number of antibodies remain in the blood circulation.The pathogen in vaccinations are dead or inactivated so they should not make people feel ill. When people are infected with live pathogens, they usually feel ill for a while because it normally takes a few day for the activated lymphocytes to produce the correct antibody for the pathogen and destroy it. This is called primary immune response. If the person is later infected with the same pathogen, they already have the correct antibodies present antibodies present and the lymphocytes can rapidly produce more and destroy the pathogen before it makes the person feel ill. This is secondary immune response.