Specific antibody deficiency with normal immunoglobulin concentrations and normal numbers of B cells

Specific antibody deficiency with normal immunoglobulin concentrations and normal numbers of B cells (SAD) is a primary immunodeficiency disorder characterized by an impaired ability to produce protective antibodies against specific pathogens — particularly polysaccharide-encapsulated bacteria — despite having normal total levels of immunoglobulins (IgG, IgA, IgM) and normal B cell counts. This condition is also referred to as specific antibody deficiency (SAD) or selective antibody deficiency with normal immunoglobulins. Because quantitative immunoglobulin levels appear normal on routine testing, the diagnosis is often delayed and requires functional assessment of antibody responses, typically by measuring antibody titers before and after vaccination with unconjugated pneumococcal polysaccharide vaccine. The immune system's inability to mount adequate responses to polysaccharide antigens primarily affects the respiratory tract, leading to recurrent sinopulmonary infections including sinusitis, otitis media, bronchitis, and pneumonia. Patients may also experience recurrent infections of the gastrointestinal tract. Over time, untreated or poorly managed recurrent infections can lead to complications such as bronchiectasis and chronic lung disease. The condition predominantly affects the humoral (antibody-mediated) arm of the immune system. Treatment depends on the severity and frequency of infections. Mild cases may be managed with prophylactic antibiotics and close monitoring. More severe cases, particularly those with recurrent serious infections or evidence of end-organ damage, may benefit from immunoglobulin replacement therapy (intravenous or subcutaneous). Conjugate pneumococcal vaccines may also be tried, as some patients can respond to protein-conjugated polysaccharide antigens even when they fail to respond to unconjugated polysaccharide vaccines. In some children, the condition may improve or resolve with age as the immune system matures, though in others it persists into adulthood.

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