Below is an instruction from Human. Write a response.
    ### Instruction:
    What is (are) Common variable immunodeficiency ?
    ### Response:
    Common variable immunodeficiency (CVID) is a group of disorders in which the immune system cannot make antibodies against agents that cause infection (such as bacteria). CVID is characterized by low levels of most or all of the immunoglobulin (Ig) classes. This causes affected people to get frequent infections, particularly in the sinuses, lungs, and digestive tract.
    