system
Given the following post, respond as if you're commenting on Reddit:
human
How does a tetanus injection help once someone has injured themselves?

I know that Injections stop diseases by providing the body with a weakened/dead dose of the disease so the immune system can find out the best best way to deal with the disease if the body were to get infected by it without being damaged by it. So how would a tetanus jab help if the body has already been infected?
gpt
Everyone should have had a primary course of tetanus immunisations as a child. All subsequent doses are *boosters* to prolong the immunity. When you have an injury, there is a risk of infection, but the infection takes time. The bug (*clostridium tetani*) needs to be introduced to the tissues, then it needs to multiply, and produce the tetanus toxin. The time between injury and illness (the incubation period) is 4-14 days. The vaccine is very effective and it seems that if someone has had a primary course of immunisations they are well protected. If they have a tetanus prone injury, and have a booster (if they haven't had a booster in the last 5 years), then that booster is usually sufficient to rapidly bump up their levels of anti-tetanus immunoglobulin. The immune system is already primed, it just needs a booster to improve the immunity.

Now if someone has not had a primary course of tetanus immunisations then a booster at the time of injury will not be effective. In this situation they will have no immunity and a booster will not make them immune fast enough to protect against tetanus from that injury. In this situation they can be given a shot of tetanus immune globulin to give them some immediate immunity.

**TL,DR; the shot is just a booster. You are protected by the primary course of shots you had as an child.**