Posted 4/5/2007 5:09 PM (GMT -6)
The lymph nodes are the collecting point for lymphocytes and b cells, both of which take up antigens (bacteria, allergens, viruses, etc) from the body and use the lymph node to activate more white blood cells. Unless your exposure to an allergen is only occuring on one side of your body (like you lotion up side of your neck, and nothing else), near to the one lymph node, then it should be enlarging all of the local lymph nodes.
If your allergy is to dust or something, then it won't just enlarge one lymph node. What you describe is known as Localized lymphadenopathy. I assume your dad checked for lesions or other things that may have caused it (a cut in the area or rash or something.) Assuming he found nothing, you should watch it for 3-4 weeks. If it doesn't go away, and you don't have mono/HIV/Syphilis/lymphoma, then you may want to go and see a Dr. Or a specialist. You may need a biopsy of the lymph node.
Here's the American Academy of Family Physicians differential diagnosis information. Give it a read. http://www.aafp.org/afp/981015ap/ferrer.html
Hope you feel better.