Fever and cold are not diseases but are only symptoms of the immune system putting up a brave fight against a foreign body. Likewise, sudden swelling up in the area where a forest Ant has bitten is also a symptom that our immune systems are working just fine. The lymph nodes play a great role in identifying a foreign body when it enters the body. A fitting response is given to the bacteria by the lymphocytes. It is a wonderful fact that the immune system has memory power. If the same bacteria or virus that attacked the body repeats its attack again, the immune system would immediately recognize this and would fight it off powerfully this time.