Choose the letter A, B, C or D to complete the passage belowAn allergy is an

Choose the letter A, B, C or D to complete the passage below
An allergy is an immune system response __________ (6) a foreign substance that's not typically harmful to your body. These foreign substances are called allergens. They can __________ (7) certain foods, pollen, or pet dander.
Your immune system's job is to keep you healthy by fighting harmful pathogens. It does this by attacking anything it thinks could put your body in danger. Depending on the allergen, this response may involve inflammation, sneezing, or a host of __________ (8) symptoms.

Your immune __________ (9) normally adjusts to your environment. For __________ (10), when your body encounters something like pet dander, it should realize it's harmless. In people with dander allergies, the immune system perceives it as an outside invader threatening the body and attacks it. Allergies are common. Several treatments can help you avoid your symptoms.