hi, welcome to health care magic. Lymph nodes are found in different parts of the body, their job is to respond to infection at different points. They are found in the neck, under the chin, at the elbows on the inner side, in the groin and under the arms. They are responsible for fighting infection and are an important part of our immune system, they would generally fight infections in the area that they are located. That is why cervical lymph nodes tend to become enlarged when children gets colds, because the colds involve the nose, ears, throat ( and upper chest in some cases) and cervical lymph nodes contain cells that produce proteins that capture and fight viruses and other organisms in that area. The fact that the nodes were mobile and non tender is reassuring. Having said that lymph nodes can become enlarged for reasons other than fighting distant infections which is why the doctor would have done the blood tests. The ESR test which was done is a non specific marker for infection or inflammation. It can tell you that something is going on in the body but it cannot tell you what. The worse the infection or inflammation the higher the value. It would go up in a person with a cold and be expected to fall after the cold resolves if the person does not have any other illnesses. The platelet count can also rise in an acute illness and be expected to normalize if no other problems exist. The size of the lymph node (in the absence of any other problems) would be expected to decrease, this can be monitored by you and your doctor. Your doctor may consider repeating the blood test when you son is well to make sure that it was the cold that caused the increase. Unfortunately because children get colds so often it is sometimes difficult to find a long enough well period. I hope that this information is helpful to you