A 7-year-old boy with a history of asthma presents for an emergency appointment after a fall in which he struck his anterior maxilla and lacerated his lips. The maxillary central incisors are mobile with bleeding from the gingival sulcus, and the maxillary lateral incisors are mobile without bleeding. During the initial exam he develops acute dyspnea. Which of the following immunoglobulin classes is MOST active in this patient's respiratory difficulties?