Antibodies — Structure and Classes

Which antibody class is the most common in mucosal secretions (saliva, tears, colostrum, and respiratory secretions) and provides the first line of defense against pathogens at mucosal surfaces?

A 28-year-old man with a history of recurrent Neisseria meningitidis meningitis (2 episodes in 18 months) presents for evaluation. Serum complement levels are normal. Serum immunoglobulin levels are normal. Flow cytometry shows normal B and T cell counts. Specific antibody response to meningococcal polysaccharide vaccine is absent. What is the most appropriate immediate next step in management?

A 32-year-old woman presents to the immunology clinic with a 6-month history of recurrent sinusitis and bronchitis. Serum protein electrophoresis shows a monoclonal spike. Flow cytometry reveals CD19+ B cells producing only kappa light chains. Serum IgM levels are markedly elevated (850 mg/dL; normal <200), while IgG and IgA are normal. A bone marrow biopsy shows 40% plasma cells. Which antibody class is predominantly elevated, and what is the structural basis for its poor opsonization compared to IgG?

A 28-year-old primigravida at 32 weeks of gestation is found to have anti-D antibodies (Rh antibodies) in her serum at a titer of 1:64. She is Rh-negative and has no history of transfusion. Obstetric ultrasound shows fetal hydrops. Which antibody class is responsible for hemolytic disease of the newborn in this scenario, and what is the structural feature that allows transplacental passage?

Which immunoglobulin class is primarily responsible for opsonization and complement activation in the primary immune response?