Antibody deficiencies — formative SAQs

SAQ 1 (10)

A nine-month-old boy is referred for investigation of recurrent infection. He has had three episodes of otitis media, one admission for lobar pneumonia requiring intravenous antibiotics, and a recent episode of septic arthritis. His maternal uncle died in infancy of "an infection." On examination his tonsils are small and there are no palpable lymph nodes. His total IgG, IgA and IgM are all well below the age-adjusted reference range, and a lymphocyte subset shows absent CD19-positive B cells. [3] [2]

a) What is the most likely diagnosis, and which gene is implicated? (2 marks) [3]

b) Explain why the presentation occurs at nine months rather than in the neonatal period. (2 marks) [3]

c) Outline the further investigations needed to confirm the diagnosis and to plan management. (3 marks) [2] [6]

d) Discuss the principles of long-term management, including treatment, family implications, and prognosis. (3 marks) [3] [5]

SAQ 2 (10)

A thriving two-year-old girl has had six upper-respiratory infections and two proven pneumonias in the past year. Her total IgG is below the age-adjusted reference range, with normal IgA and IgM; her full blood count is normal and she is growing along the 50th centile. A pneumococcal polysaccharide vaccine response test shows a preserved antibody rise. [6] [4]

a) What is the most likely diagnosis, and what feature most strongly distinguishes it from common variable immunodeficiency? (3 marks) [6] [4]

b) Justify why immunoglobulin replacement is not indicated at this stage, and describe the expected natural history. (3 marks) [6] [5]

c) Outline a follow-up plan, including the triggers that would prompt escalation of investigation or treatment. (2 marks) [6] [4]

d) State two causes of secondary hypogammaglobulinaemia that must be excluded before a primary defect is confirmed. (2 marks) [6] [10]