Ah yes, this is something I've asked our pedia about din kasi my son needed boosters for tetanus at school! Basically, some vaccines give long-lasting protection, like measles, you're mostly good for life, but others like tetanus and pertussis need reminders every few years because those diseases spread so fast na ang immune system needs a "heads up" to catch them in time, otherwise by the time your body remembers what to do, you're already sick. It's like your antibodies are sleeping guards, and boosters wake them up to say "hey, stay alert!"