system
Given the following post, respond as if you're commenting on Reddit:
human
I was just diagnosed with the first stage of Lyme's Disease for the third time in five years.  Each is from an independent tick bite, but why didn't my immune system catch on after the first time?
gpt
Bacteria are covered with proteins which the immune system can recognize. Within a strain, the bacteria tend to have the same surface proteins--meaning that once you have been exposed, your immune system makes antibodies, and you are often immune. 

Bacteria have evolved mechanisms to evade the immune system. These mechanisms allow them to reinfect you even though you have antibodies to some of their surface proteins. Borrelia burgdorferi, the bacteria that causes Lyme disease, undergoes a process called 'antigenic variation'. This means that the bacteria is constantly changing its surface protein. Basically, it started with protein A and your immune system now makes antibodies against A; the bacteria mutates its surface proteins just enough to make them type B, which your immune system hasn't made antibodies to yet. So, by changing its surface proteins, the bacteria is able to escape detection by the immune system, and you can get Lyme disease multiple times.