What is specific antibody deficiency (SAD) or specific polysaccharide antibody deficiency (SPAD) associated with hypogammaglobulinemia?

Updated: Dec 26, 2018
  • Author: Elizabeth A Secord, MD; Chief Editor: Michael A Kaliner, MD  more...
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Specific antibody deficiency (SAD) or specific polysaccharide antibody deficiency (SPAD)

  • Though the prevalence of this condition is not known, it is occasionally found in patients with recurrent sinopulmonary infections. SAD is characterized by total levels of IgG, IgA, and IgM within the normal range, but with an inability to make appropriate quantities of specific antibodies and/or to retain memory of polysaccharide vaccines. As with most humoral immune deficiencies described, recurrent sinopulmonary infections are the hallmark.

  • No consensus exists as to the titer or number of pneumococcal serotype antibody responses that should be elicited in order to fit into this disorder. [7]

  • Age must be considered when entertaining this diagnosis. While no reliable age-adjusted criteria for polysaccharide response exists, the general guideline is that the younger the patient, the fewer the responses. The diagnosis should not be assigned to children younger than 2 years, because IgG2, IgA, and specific polysaccharide responses usually develop more slowly than other types of antibody response.

  • A positive response is usually defined as a titer to a specific serotype greater than 1.3 mg/mL or a 4-fold increase in preimmunization titers. [7] Some authors suggest that at least 3 serotypes showing specific antibody levels ≥2 µg/mL probably represents a normal antibody responsiveness, while others suggest that 9 out of 12 serotype responses is considered normal.

  • In patients who have already been vaccinated with conjugated pneumococcal vaccines, the actual response may be difficult to determine because prevaccination titers were not available to determine antibody increases, and no consensus exists about what values constitute protective titers in patients who only have postvaccination titers. Meningococcal and typhoid vaccines are other potential antigens that can be used to assess antibody responses. [8, 7] Antibody responses to polysaccharide antigens (eg, unconjugated pneumococcal polysaccharide vaccine) in normal children younger than 2 years are often poor, which is why protein conjugate vaccines are usually used in this age group.

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