What is the difference between an allergy and asthma?

Location: Both allergies & asthma involve inflammation of the respiratory tree, but allergies tend to involve the upper parts while asthma affects the lower. They are often seen together, and it is estimated that about 75% of asthmatics also have allergies; that makes sense since the respiratory tree is connected, what affects one area can affect the other. Both are treated with anti-inflammatory medicines.

Quite a bit: Two different conditions with some overlap. Asthma is an increased responsiveness of the bronchial tubes that results in difficulty breathing, cough and wheezing. Allergy can trigger asthma, but that is only true for 10% of adult asthma (higher in children). Allergy is a reaction to a substance that can manifest as skin rash, runny nose, itching, cough ..Etc.

Difference...: Asthma is recurrent episodes of airway inflammation and bronchospasm. Allergy is an abnormal reaction to what are usually innocuous triggers. Asthma episodes can be caused by allergic reactions, among other things. And, there are other allergic reactions besides asthma-like episodes.

One causes the other: The two walk together. Asthma commonly is caused by allergies affecting the lower airways - the bronchial tubes tighten, become swollen and become congested with excess mucus. Asthma can also be triggered by other things, such as exercise, smoke, viruses etc., but by controlling allergies, asthma and asthmatic reactions are much less likely to occur.