Which of the following is the most common form of primary immunodeficiency?

Selective IgA deficiency (less than 15mcg/ml) is the most common type of primary immunodeficiency. Individuals develop recurrent acute and chronic respiratory infections that may lead to bronchiectasis. These patients are at an increased risk of drug allergies, atopic disorders and autoimmune diseases. Ref: Harrison's Principles of Internal Medicine, 18th edition, chapter 316.