If you suffer from allergies, that does not mean that you have a generally weakened immune system; it simply means that your immune system is not functioning properly as it is creating antigens to attack normally harmless substances (pet dander, pollen, dust mites, etc.). Having allergies is entirely an immune-related disease, but many studies have shown that those who suffer from allergies typically have a stronger immune system against other diseases or infections. Allergy symptoms are just your immune system being hypersensitive and responding too strongly to allergens. That being said, suffering from allergies long-term without being able to properly relieve the symptoms can lead to a weakened immune system, making it more difficult to fight off other viruses and bacteria.