Vaccination and Herd Immunity

Vaccination is one of the most important public health measures in history, preventing millions of deaths each year. This lesson covers the principles of vaccination, types of vaccine, herd immunity, and the ethical considerations surrounding vaccination programmes, as required by the Edexcel A-Level Biology (9BI0) specification.

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What is Vaccination?

A vaccine is a preparation containing antigens from a pathogen (or a modified form of the pathogen) that stimulates an active immune response without causing the disease.

How Vaccines Work

1. The vaccine introduces antigens into the body.
2. The antigens are recognised by specific B cells and T cells (via antigen-presenting cells).
3. Clonal selection and clonal expansion occur — the correct lymphocytes are activated and multiply.
4. Effector cells (plasma cells, cytotoxic T cells) destroy the antigens.
5. Memory cells (memory B cells and memory T cells) are produced and persist in the body.
6. Upon subsequent exposure to the real pathogen, memory cells mount a rapid, strong secondary immune response, preventing the disease from developing.