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How vaccines work
Vaccines use a harmless form of a pathogen, or some component of it, to induce a protective immune response involving one or both arms of the immune system: humoral and/or cell-mediated immunity.
Humoral immunity is based on antibodies and the B cells that produce them. Antibodies are proteins that recognise a specific target, usually part of the surface of a protein. 'Neutralising' antibodies, which normally bind to the outside of a virus, can play an important role in fighting off viral infections.
Cytotoxic (cell-killing) T-cells play a major role in the other arm of the immune system, cell-mediated immunity. These are also called CD8 T-cells because of a protein, CD8, present on their surface. Cytotoxic T-cells are able to destroy virally infected cells, identified by the presence of very small fragments of viral proteins (epitopes) which are displayed on their surface. CD4 ('helper') T-cells also recognise fragments of viruses, displayed on the surface of specialised 'antigen presenting cells'. These produce proteins which activate B cells and cytotoxic T cells.
When the immune system is activated by vaccination, memory T-cells and memory B-cells are produced. These cells enable a rapid and effective immune response when the pathogen itself is encountered, preventing infection or disease.
Another term that is sometimes used is 'mucosal immunity'. This refers to humoral or cellular immune responses which are concentrated at the mucosal surfaces where most HIV transmission takes place, such as the vagina, urethra and rectum, and inside the mouth. Sometimes, for example, it appears that HIV antibodies can be produced at mucosal surfaces without being detectable in the blood and without evidence of continuing infection. This suggests the possibility of vaccines that could specifically induce immune responses acting only where they are most needed, to prevent sexual transmission or transmission of HIV through breast milk.