Vaccination and immune memory

Explanation

A vaccine presents the immune system with an agent resembling a pathogen, or relevant parts of it, so that its antigens can be recognised. It stimulates a response without requiring the person to experience the disease in its usual form.

Appropriate white blood cells respond by producing antibodies specific to those antigens. Antibodies bind to their matching antigens and help the immune system deal with the pathogen. This specificity is why one vaccine does not automatically protect against every infectious disease.

Some of the responding cells become memory cells. If the matching pathogen enters later, the immune system can respond more rapidly and produce antibodies sooner, often in greater amounts. The pathogen can therefore be controlled more quickly, reducing the likelihood or severity of disease.

Protection takes time to develop and varies with the vaccine and pathogen. Vaccination does not guarantee that infection is impossible. A vaccination graph should distinguish the first immune response from the later response; the antibody concentration does not instantly jump to its maximum when a vaccine is given.

Use this definition in an answer: a vaccine contains a harmless (dead or weakened) form of a pathogen, or parts of it, that carries the pathogen's antigens. Its purpose is to stimulate the body to make specific antibodies and memory cells without causing the disease, so that the body responds quickly if the real pathogen enters later.

Read antibody graphs with figures. In an invented example, after a first dose the antibody level rises slowly from 0 to a peak of 12 arbitrary units at day 14, then falls. After a second exposure to the same antigen at day 60, the level starts rising within 2 days and peaks at 120 arbitrary units at day 65. The second peak is 120 / 12 = 10 times higher and is reached in 5 days instead of 14, because memory cells divide quickly and produce antibodies sooner.

Influenza vaccines are given every year. The influenza virus mutates often, so its surface antigens change. Antibodies and memory cells made against last year's antigens no longer fit the new antigens, so the new strain needs a new vaccine. The virus has not become resistant to the vaccine: it now carries different antigens.

Vaccinating most of a population also protects people who are not vaccinated. When most people are immune, an infected person is unlikely to meet someone who can catch the pathogen, so it spreads to fewer people and chains of transmission break. Babies too young to be vaccinated and people with weak immune systems are protected because the pathogen rarely reaches them. This is called herd immunity; an answer must explain it in this way, not just name it.