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How Vaccines Work

Immune Response to a Vaccine

Vaccines help the body acquire immunity against many disease-causing germs and cancers. A vaccine contains a killed or weakened form or derivative of an infectious germ. The vaccine has little or no disease-causing ability, but its presence in the body still provokes an immune response. This activates various immune cells that learn from the vaccine to recognize and destroy the germ.

  1. A vaccine activates various immune cells because it contains part of the germ, called an antigen, that stimulates the body's immune response. An antigen by itself or in a vaccine has little to no disease-causing ability.
  2. The first immune cells that encounter the vaccine are called antigen-presenting cells. Each antigen-presenting cell digests an antigen, then displays on its surface a small piece of antigen that can be recognized by T cells.
  3. When antigen-specific helper T cells encounter an antigen-presenting cell, they become activated and send a chemical messenger to other immune cells—e.g., B cells and killer T cells. The chemical messenger helps these immune cells become activated.
  4. Once stimulated by the antigen and the chemical message from the helper T cells, the B and killer T cells divide and transform into specialized immune cells that fight back against that specific antigen. Also, a small but important fraction of the B and T cells transform into memory cells that react quicker when they encounter the same antigen again.

Immune Response after Vaccination

  • An exposure to a germ after vaccination stimulates the memory B and memory T cells, which recognize the antigen from the germ and respond quickly and effectively to prevent disease.
  • Activated memory B and memory T cells remember the antigen from the vaccine and respond faster and more efficiently against future infection by the same antigen.
  • Plasma B cells produce a molecule called an antibody that recognizes and binds to the antigen on the germ or cells infected with the germ.
  • Antibodies bind to antigens on the germ to prevent infection, as well as to the cells infected with the germ to mark them for killing.
  • Memory T cells encounter and recognize the antigen displayed on the surface of the antigen-presenting cells that initiate the immune response against the germ. This activates memory T cells.
  • Activated killer T cells bind to and destroy any cells that contain the germ's antigen.