Immunity, vaccines and herd immunity

Organisms (Interaction and interdependence) · Defence against disease · note 5 of 8

Spec C3.2.10, C3.2.16, C3.2.17Download all Defence against disease notes (PDF)Download all notes (PDF)11 pages

Immunity, vaccines and herd immunitySpec C3.2.10, C3.2.16, C3.2.17

In short

Immunity is the ability to eliminate an infectious disease from the body, due to the long-term survival of memory cells that can make the specific antibodies needed. Vaccines contain antigens, or DNA or RNA coding for antigens, and stimulate immunity to a specific pathogen without causing the disease. If enough of a population is immune, herd immunity greatly impedes transmission.

Immunity and memory cells

Immunity is the ability to eliminate an infectious disease from the body. It is due to the long-term survival of lymphocytes capable of making the specific antibodies needed to fight the infection: these are memory cells. On a second infection by the same pathogen, memory cells are activated quickly and divide to form plasma cells, so antibodies are produced faster and in greater quantities. The pathogen is usually eliminated before symptoms develop.

Graph of antibody concentration in the blood against time, with a small slow primary response after first exposure to an antigen and a much faster, larger secondary response after second exposure to the same antigen. — The secondary response is faster and larger because memory cells survive from the first exposure.

Vaccines and immunization

A vaccine contains antigens, or nucleic acids (DNA or RNA) with sequences that code for antigens. Antigens may be on killed or weakened pathogens, or be purified pathogen proteins. With an mRNA vaccine, the person's own cells use the RNA to make the antigen. Either way, the vaccine stimulates a primary immune response and the development of memory cells, so the person gains immunity to a specific pathogen without causing the disease. Immunization is the process of making a person immune, usually by vaccination.

Herd immunity

Members of a population are interdependent in building herd immunity. If a sufficient percentage of a population is immune to a disease, an infected person is unlikely to meet a susceptible person, so transmission is greatly impeded and epidemics are prevented. This protects people who cannot be vaccinated, such as newborn babies or people with weakened immune systems. The more transmissible the pathogen, the higher the percentage that must be immune.

Exam tip:

Nature of science: vaccines are tested rigorously and the risks of side effects are minimal but not nil. The media often report research while evaluation by other scientists is still happening, so distinguish pragmatic truths from certainty.

Written and checked against the IB Biology SL specification · Updated October 2026

Frequently asked questions

How do vaccines work?

Vaccines contain antigens, or DNA or RNA coding for antigens, from a specific pathogen. They trigger a primary immune response in which B-cells are activated and memory cells are produced, without causing the disease. If the real pathogen infects later, memory cells respond quickly, producing antibodies before the person becomes ill.

Why can't antibiotics kill viruses?

Antibiotics work by blocking processes found in bacteria but not in eukaryotic cells, such as cell wall synthesis or protein synthesis by 70S ribosomes. Viruses have no metabolism of their own, so there is nothing for antibiotics to block. They replicate using the host cell's processes, which antibiotics do not target.

What is the difference between innate and adaptive immunity?

The innate immune system responds to broad categories of pathogen and does not change during an organism's life; phagocytes are part of it. The adaptive immune system, based on lymphocytes, responds specifically to particular pathogens and builds up a memory of pathogens encountered, so later responses are faster and more effective.

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