Final answer:
The intestinal structures made up of aggregated lymphocytes and macrophages are Peyer's patches, which are part of MALT and vital for immune defense in the small intestine.
Step-by-step explanation:
The intestinal structures consisting of protective aggregated nodules of lymphocytes and macrophages are called Peyer's patches. These are part of the mucosa-associated lymphoid tissue (MALT) and are crucial for initiating immune responses in the small intestine.
Peyer's patches contain specialized cells known as M (microfold) cells that collect antigens from the intestinal lumen, allowing the immune system to detect and respond to potential pathogens. The dendritic cells present antigens to T cells and B cells within these patches to mount a strong immune response. Furthermore, Peyer's patches produce immunoglobulin A (IgA) and other pathogen-specific antibodies that enter the intestinal lumen, contributing to the body's defense mechanisms against ingested microbes. These patches are particularly prominent in the distal ileum and are most active during youth, reflecting the high demand for immune surveillance in younger individuals.