Thank you for your query on Healthcare Magic,
Getting frequent colds is not a symptom or sign of low immunity.
Frequent colds are due to allergic rhinitis
, allergic environment and repeated exposures to viruses.
But loss of weight and loss of appetite are significant and needs evaluation by a physician.
There are many investigations are there, but very difficult for a common man to go for and interpret their results and unneccessarily they cause apprehension.
So consult your Physician, he will take a proper history of your problems, and depending on complete examination, he will plan investigations and he will correlate them with your symptoms and examination findings.
Hope I have answered your query.