Lymphatic fluid gathers from the tissues and enters the valved lymphatic ducts. It is the valves which make sure the lymph flows the right way. On its way back to the blood system, the lymph passes through glands called lymph nodes, in places like the back of the neck, the armpits and the groin. Lymph nodes swell in the region of an infected body part. This acts to keep the infection from spreading. It prevents or reduces the pathogens getting into the general blood circulation and reaching other parts of the body.