Lymph nodes are small, bean-shaped structures that are part of the lymphatic system. They play a crucial role in the body's immune response by filtering lymph fluid, which contains white blood cells that help fight infections. Lymph nodes are distributed throughout the body, often found in clusters in areas like the neck, armpits, and groin. When the body detects an infection or disease, lymph nodes can become swollen as they produce more immune cells to combat the threat. This swelling is often a sign that the body is actively fighting an illness, making lymph nodes important indicators of health.