A vaccine shows the immune system a harmless version or piece of a germ. The body makes antibodies and memory cells, so a later real infection is stopped faster or causes milder disease.
Building memory
A vaccine triggers a first immune response with activation of T and B cells. The memory cells that remain respond within days on real exposure, often before symptoms start.
Types of vaccine
Live attenuated vaccines use a weakened germ, as in measles, mumps, rubella and varicella. Inactivated, subunit and toxoid vaccines use a killed germ, a piece of it or a disabled toxin, as in the flu shot, hepatitis B and tetanus, and mRNA vaccines give cells instructions to make one germ protein.
Doses and boosters
Many vaccines need several doses or later boosters to build and keep enough protection. Some contain an adjuvant that strengthens the response.
Community immunity
When most people in a community are immune, a germ cannot spread easily. This protects babies and people who cannot be vaccinated.
When to avoid live vaccines
Live vaccines are generally avoided in pregnancy and in people with weakened immune systems, because the weakened germ can still cause disease.
At the bedside
Before vaccinating, check the schedule, allergies and any reason to avoid a live vaccine, and observe the patient for about 15 minutes afterward. Most vaccines go intramuscularly into the deltoid in adults or the outer thigh in infants.
Related guides
Written by Making Synapses. Topic reference: OpenStax, Anatomy and Physiology 2e (no text adapted).