Below is an instruction from Human. Write a response.
    ### Instruction:
    What causes Hypersensitivity Pneumonitis ?
    ### Response:
    Repeatedly breathing in foreign substances can cause hypersensitivity pneumonitis (HP). Examples of these substances include molds, dusts, and chemicals. (Mold often is the cause of HP.) These substances also are known as antigens.
                
Over time, your lungs can become sensitive to antigens.
    