Why was the lymphatic system the most impacted by the plague?

Plague is caused by the bacteria Yersinia pestis, a zoonotic bacteria usually found in small mammals and their fleas.
People infected with Y. pestis often develop symptoms after an incubation period of one to seven days.
There are two main clinical forms of plague infection: bubonic and pneumonic. Bubonic plague is the most common form and is characterized by painful swollen lymph nodes or 'buboes'.