A 50-year-old man with end stage renal disease from long-standing diabetes mellitus receives a cadaveric renal transplant. A month later, he has increasing malaise. His urine output decreases. Laboratory studies show a rising serum urea nitrogen and creatinine. The allograft is biopsied and seen microscopically to be undergoing destruction by cells which are recognizing graft cells expressing class I HLA antigens. Which of the following cells is most likely to mediate this response?

Persons infected with HIV are shown to have a period of clinical latency of infection lasting years. During this time their risk for infection by opportunistic pathogens is not increased. However, a reservoir of virus remains within their lymphoid tissues. Which of the following cells is most important to maintain this reservoir of HIV?

A 21-year-old man with serum IgA of 22 mg/dL, IgG of 175 mg/dL, and IgM of 40 mg/dL has had a high fever with cough productive of yellowish sputum for the past two days. Auscultation of the chest reveals a few crackles in both lung bases. A chest radiograph reveals bilateral patchy pulmonary infiltrates. Which of the following inflammatory cell types will most likely be seen in greatly increased numbers in his sputum specimen?

A 31-year-old man incurs an injury in an industrial accident, with a steel H-beam striking the right side of his face, crushing the zygomatic arch and damaging the globe of the right eye. He undergoes plastic repair of bone and soft tissues and an attempt is made to salvage the eye. A month later, he has decreased visual acuity in both eyes, with choroidal thickening seen on funduscopic examination. The right eye is enucleated and shows prominent CD4 lymphocyte and macrophage infiltration of the uvea. By which of the following immunologic mechanisms has this condition most likely developed?

A neonate born at term developed tetany soon after birth. On physical examination the infant has a heart murmur. Laboratory studies show a serum calcium of 6.3 mg/dL. Echocardiography reveals a membranous intraventricular septal defect. Within the next year, this infant has bouts of Pneumocystis carinii (jiroveci) pneumonia, Aspergillus fumigatus pneumonia, and parainfluenza virus and herpes simplex virus upper respiratory infections. Which of the following abnormalities most likely explains the development of this infant's findings?