To answer the above question, the relevant sentence is: Similar in structure to a large lymph node, it acts primarily as a blood filter.
This justifies what answer for what question? Q & A:
Passage: The spleen is an organ found in virtually all vertebrates. Similar in structure to a large lymph node, it acts primarily as a blood filter. The word spleen comes from Ancient Greek (splen).

Question: Based on this passage, what is the purpose of the spleen in a human body?
a blood filter