Specific Antibody Deficiency
Also written as: SAD — Specific Antibody Deficiency
A condition in which serum immunoglobulin levels are normal but the patient fails to mount adequate antibody responses to polysaccharide antigens such as those in pneumococcal vaccines.
Full Definition
Specific antibody deficiency (SAD) is characterised by normal total immunoglobulin concentrations yet an impaired ability to produce protective IgG antibodies against polysaccharide antigens. It is diagnosed by measuring pre- and post-vaccination titres to unconjugated pneumococcal polysaccharide vaccine in children over two years of age. Because overall immunoglobulin levels appear normal, SAD is frequently overlooked in routine workups, making it a diagnostic pitfall for editors reviewing immunology reports. The condition occupies a spectrum ranging from partial to complete polysaccharide unresponsiveness. It is the most common antibody deficiency identified in children with recurrent sinopulmonary infections.
Usage
Usage note: When SAD appears as an acronym in manuscripts, editors should verify it is not confused with the psychiatric abbreviation for seasonal affective disorder; context must be explicit.
In Context
- "The immunology report confirmed specific antibody deficiency after post-vaccination pneumococcal titres remained below protective thresholds." — Outpatient immunology consultation letter
- "Editors should note that specific antibody deficiency does not present with low total IgG, distinguishing it from common variable immunodeficiency." — Clinical review article