In which portion of the small intestine are large masses of lymphoid tissue, known as aggregated lymphoid nodules or Peyer's patches, most abundant?

Answer: distal end of ileum

Explanation: Numerous lymph nodes are found in the mucosa throughout the length of the small intestine.

The Peyer's patches is a collection of 20 or 30 larger nodes situated towards the distal end of the ileum.

These lymphatic tissues are strategically located to neutralize ingested antigens