A vaccine contains antigens from a pathogen, given so that the body makes an immune response and memory cells without the person having the disease. This gives artificial active immunity.The antigens in a vaccine can be:
• live attenuated (weakened) viruses, e.g. the MMR vaccine
• inactivated (killed) viruses, e.g. the injected polio vaccine
• subunits: purified viral proteins, e.g. the HPV and hepatitis B vaccines
• mRNA coding for a viral protein, which the body's own cells then make, e.g. some COVID-19 vaccinesAfter vaccination, the antigen is taken up by antigen-presenting cells. Specific T helper cells and B cells are activated, and B cells divide to form plasma cells, which make antibodies, and memory cells. This is the primary immune response.If the real virus later enters the body, the memory cells recognise its antigens and produce a secondary immune response. This is faster and larger, with many more antibodies made, so the virus is destroyed before it can cause symptoms. A second dose (a booster) is often given to produce more memory cells.
When a high proportion of a population is vaccinated, the virus cannot spread easily, which also protects unvaccinated people. This is herd immunity.Vaccines work less well against viruses that mutate quickly. Antigenic variation, as in influenza, means the new strain's antigens no longer fit the memory cells' receptors, so a new vaccine is needed each year. It is also hard to vaccinate against HIV, because it mutates rapidly and hides as a latent provirus.
• live attenuated (weakened) viruses, e.g. the MMR vaccine
• inactivated (killed) viruses, e.g. the injected polio vaccine
• subunits: purified viral proteins, e.g. the HPV and hepatitis B vaccines
• mRNA coding for a viral protein, which the body's own cells then make, e.g. some COVID-19 vaccinesAfter vaccination, the antigen is taken up by antigen-presenting cells. Specific T helper cells and B cells are activated, and B cells divide to form plasma cells, which make antibodies, and memory cells. This is the primary immune response.If the real virus later enters the body, the memory cells recognise its antigens and produce a secondary immune response. This is faster and larger, with many more antibodies made, so the virus is destroyed before it can cause symptoms. A second dose (a booster) is often given to produce more memory cells.
When a high proportion of a population is vaccinated, the virus cannot spread easily, which also protects unvaccinated people. This is herd immunity.Vaccines work less well against viruses that mutate quickly. Antigenic variation, as in influenza, means the new strain's antigens no longer fit the memory cells' receptors, so a new vaccine is needed each year. It is also hard to vaccinate against HIV, because it mutates rapidly and hides as a latent provirus.