Specific Antibody Deficiency (SAD) (also known as Selective Antibody Deficiency or Impaired Response to Polysaccharide Antigens) Key Clinical Features Definition: Specific Antibody Deficiency (SAD) is a primary immunodeficiency characterized by normal absolute levels of total immunoglobulins (\text{IgG}, \text{IgA}, \text{IgM}) and normal \text{IgG} subclass levels, but a specific inability to produce functional protective antibodies against polysaccharide antigens (such as encapsulated bacteria like Streptococcus pneumoniae, Haemophilus influenzae type b, and Neisseria meningitidis). Clinical Presentation: Recurrent upper and lower sinopulmonary tract infections (e.g., recurrent otitis media, chronic sinusitis, bronchitis, and pneumonia) in children or adults despite normal quantitative baseline immunoglobulin levels. Diagnostic Standard: Baseline anti-pneumococcal antibody titers are measured. The patient is immunized with the 23-valent pneumococcal polysaccharide vaccine (PPSV23 / Pneumovax). Post-vaccination titers are measured 4\text{--}6 weeks later. A failure to mount a 4-fold or protective increase in titers to a majority of serotypes confirms the diagnosis. Management Strategy Prophylactic Antibiotics: Daily low-dose prophylactic antibiotic therapy (e.g., Amoxicillin or Trimethoprim-sulfamethoxazole) is often the initial management strategy. Conjugate Vaccination: Administration of pneumococcal conjugate vaccines (PCV13 / PCV20), which recruit T-cell-dependent immune responses and may elicit protective antibody responses where pure polysaccharide vaccines fail. Immunoglobulin Replacement Therapy (IgRT): Intravenous (IVIG) or subcutaneous (SCIG) immunoglobulin replacement is indicated for severe cases, refractory infections, or progression to bronchiectasis.