Primary against secondaryfrom your own schedule
Reading the two curves. IgM is the first class a plasma cell makes, so the gold line leads on a first exposure. It also fades fastest, because its plasma cells are short-lived. IgG comes later through class switching, climbs higher, and lasts far longer. On a second exposure the gold line barely moves while the purple line jumps, and that is the signature of memory.
Why a titer can look low and still mean protection. A measured antibody level only counts the antibody floating in blood right now. Memory cells are not counted at all. Someone can sit under the green line and still clear the microbe in days, because the memory cells restart the factory on contact.