I have a swollen/blocked gland under my chin and two cold sore. should i see my gp and get an antibiotic?

Yes: You should see your doctor so he can give you the best treatment for both.

Monitoring Centers: Lymphatic system absorbs/carries things too big to go into the arteries/veins like bacteria eaten by white cells and digested food. Lymph nodes "taste" the lymph for anything bad like bacteria/viruses/cancers, etc. When it detects a problem, it alerts the immune system and your body reacts. They enlarge in response to the inflammation they create when they react. They shrink when all is well.