Include tonsils, adenoids,
lymph nodes, spleen and other tissues. Act as the body's filtering system, trapping invaders(i.e., foreign particles from bacteria, viruses, etc.) and presenting them to squadrons of immune cells that congregate there. Within these lymphoid tissues, immune activity is concentrated in regions called
germinal centers, where the thread-like tentacles of
follicular dendritic cells (
FDCs) form networks that trap invaders. See also
Follicular Dendritic Cells;
Lymph Nodes.