A vaccine gives artificial active immunity. It contains antigens from a pathogen, introduced deliberately so that the body makes a primary immune response and memory cells without the person suffering the disease.Vaccines may contain:
• live attenuated (weakened) pathogens that cannot cause disease, for example the MMR vaccine
• inactivated (killed) pathogens, for example the injected flu vaccine
• toxoids: toxins made harmless, for example the tetanus and diphtheria vaccines
• subunits: isolated antigens, often made by genetically modified microorganisms, for example the hepatitis B vaccine
• mRNA coding for an antigen, which body cells use to make the antigen, as in some COVID-19 vaccines.Most vaccines are injected, but some are given by mouth (oral polio vaccine) or as a nasal spray (the children's flu vaccine).How vaccination works:
1. Antigen-presenting cells present the vaccine antigens to T helper cells.
2. Complementary B cells and T cells are selected and undergo clonal expansion.
3. Plasma cells make antibodies, and memory cells are produced.
4. If the real pathogen enters later, the memory cells produce a rapid secondary response that destroys it before symptoms develop.Some vaccines need a booster dose. This second exposure triggers a secondary response that produces even more memory cells and higher antibody levels, giving longer protection.
Vaccines are less effective against pathogens that change their antigens, such as influenza, or that hide inside cells and change form, such as Plasmodium.
• live attenuated (weakened) pathogens that cannot cause disease, for example the MMR vaccine
• inactivated (killed) pathogens, for example the injected flu vaccine
• toxoids: toxins made harmless, for example the tetanus and diphtheria vaccines
• subunits: isolated antigens, often made by genetically modified microorganisms, for example the hepatitis B vaccine
• mRNA coding for an antigen, which body cells use to make the antigen, as in some COVID-19 vaccines.Most vaccines are injected, but some are given by mouth (oral polio vaccine) or as a nasal spray (the children's flu vaccine).How vaccination works:
1. Antigen-presenting cells present the vaccine antigens to T helper cells.
2. Complementary B cells and T cells are selected and undergo clonal expansion.
3. Plasma cells make antibodies, and memory cells are produced.
4. If the real pathogen enters later, the memory cells produce a rapid secondary response that destroys it before symptoms develop.Some vaccines need a booster dose. This second exposure triggers a secondary response that produces even more memory cells and higher antibody levels, giving longer protection.
Vaccines are less effective against pathogens that change their antigens, such as influenza, or that hide inside cells and change form, such as Plasmodium.