Question

lymph helps us fight against infection

Answer 1

In an infection in an arm, the firsts lymph nodes that lymphatic vessels will drain to ... highly specific proteins that recognize the exact microbe that you are in fight and then bind to it.

Answer 2

When a microbe invade your skin in a cut, for example, a group of cells that act as a "police" patrolling the tissues (macrophages, dendritic cells) can recognize foreign cells and substances and then activate the innate response, it will recruit and activate defense cells that swallow & kill microbes and throw a lot of toxic substances that could harm them, but your own tissue, too.