Although there is evidence in the literature that gammaglobulin replacement therapy can lead to a reduction in the prevalence of pulmonary infection and improved lung function, there is no published study to guide immunologists regarding the use of spirometry in titrating IG therapy to assist in the management of immunodeficiency patients with regards to gammaglobulin replacement therapy. The investigators propose to study the use of spirometry to identify patients that could potentially benefit from an increase in IGRT. The investigators will identify 22 common variable immune deficiency (CVID) study subjects on stable IGRT replacement therapy equivalent to 0.40 to 0.60 gm/kg per 4 weeks who have evidence of mild to moderate obstruction as assessed by an FEF25-75% between 50% and 80% of predicted. Patients who are on Hizentra will be preferentially recruited. Of these 22, 11 will be identified at random and treated for 6 months at their current dose (control population). The remaining 11 study subjects (treatment group) will have their level of IGRT increased by the equivalent of 0.05 gm/kg in dose per 4 weeks, adjusted for bioavailability as per manufacturer's instructions. On average, rounded up to the nearest gram, this will typically increase their dose of Hizentra by 2 gm per week.