Let's say first you have been exposed to a germ which has "antigen A". You get sick, and your immune system has reacted to this antigen and developed an immune response and memory cells. And finally, you are healed.
Then, a few days later. You went to your sister's school and got exposed again to the germ with "antigen A", and another germ which has "antigen B". You didn't get very sick, you just had a slight sore throat.
The reason why you didn't get very sick for the second time is that your immune system reacts better when it is exposed to an antigen for a second time, due to the memory cells (that was the case for "antigen A"), so the symptoms didn't appear.
So, the sore throat came from the exposure to the new germ with the "antigen B". the immune system reacted weakly to it because if there weren't memory cells for this antigen, that's why the symptoms have appeared.
It should be pointed out that the memory cells are specific to an antigen, and that's when the principle of the vaccines came out.
