How can a vaccine prepare immune memory?

A vaccine presents the immune system with information related to a particular germ or its harmful products. That information includes antigens: features immune cells can recognize. Different vaccines supply antigens in different forms, or provide instructions that let cells briefly make an antigen. The encounter can start a specific immune response without the person having to experience the disease itself.

Cells that recognize the antigen can become activated and multiply. B cells can develop into cells that make antibodies, proteins that bind particular features. T cells can help coordinate responses or act against infected cells. Recognition depends on matching molecular features, rather than on immune cells learning a picture of an entire germ.

After the initial response settles, some cells persist as memory cells. Other cells can continue supplying antibodies. If a later encounter contains a matching antigen, memory cells can help build a response more readily than an entirely new encounter. The drawing follows part of the B-cell story; T-cell memory is also important.

Immune memory does not form instantly, and its strength and persistence vary. Antigens can also change between strains of a germ. These features help explain why protection is specific and never perfect. The sequence shows preparation for a later encounter, rather than a promise that every exposure will be blocked.