What is the antigen in the high-prevalence series, specifically the soluble form of Tamm-Horsfall glycoprotein, found in saliva and urine, that reacts at the Anti-Human Globulin (AHG) phase?

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Sda (Sid) is the Antigen in the High-Prevalence Series (901) Found in Saliva and Urine

The Sda (Sid) antigen is the high-prevalence antigen in the 901 series that is found in saliva and urine as a soluble form of Tamm-Horsfall glycoprotein, and produces characteristic refractile, mixed-field reactions at the AHG phase.

Characteristics of Sda Antigen

Biochemical Properties

  • Sda is a soluble form of Tamm-Horsfall glycoprotein (also known as uromodulin) 1
  • Tamm-Horsfall glycoprotein is uniquely produced in the kidney by tubular cells of the thick ascending limb of the loop of Henle and early distal convoluted tubules 1
  • It exhibits bidirectional secretion - both in the urine and in the renal interstitium/circulation 1
  • The protein can exist in two forms:
    • A highly polymerizing form that is apically excreted in the urine and generates filaments
    • A non-polymerizing form that retains a polymerization inhibitory pro-peptide 1

Serological Characteristics

  • Sda belongs to the high-prevalence series (901) of blood group antigens
  • The corresponding antibody typically reacts at the Anti-Human Globulin (AHG) phase
  • It produces a characteristic refractile, mixed-field reaction pattern
  • Similar to other high-prevalence antigens like YTGT in the Yt blood group system, antibodies against Sda can cause clinically significant transfusion reactions 2

Detection and Testing Methods

Laboratory Methods

  • Indirect immunofluorescence (IFL) is a preferable technique for routine autoantibody testing 1
  • Enzyme sensitivity patterns can help identify Yt system antigens, which share similarities with Sda 2
  • Neutralization studies using soluble recombinant proteins can confirm specificity 2
  • Tamm-Horsfall glycoprotein can be isolated from urine using:
    • Salt-precipitation method (Tamm & Horsfall method) - adding NaCl to urine up to 0.58 mol/L
    • Diatomaceous earth filter (DEF) method - where THP is selectively trapped due to its gelation/aggregation tendency 3

Sample Sources

  • Sda antigen is found in:
    • Saliva
    • Urine (as the soluble form of Tamm-Horsfall glycoprotein)
    • Kidney tissue (primarily in the thick ascending limb of Henle's loop)

Clinical Significance

Transfusion Medicine

  • Antibodies against high-prevalence antigens like Sda can cause significant transfusion reactions including:
    • Rigors
    • Tachycardia
    • Hypotension 2
  • Management of patients with antibodies to high-prevalence antigens may require:
    • IVIG and steroids to mitigate reactions
    • Identification of compatible rare blood units 2

Biochemical Role

  • Tamm-Horsfall glycoprotein (the source of Sda) plays important roles in:
    • Regulating salt reabsorption along the thick ascending limb
    • Interacting with renal outer medullary potassium channel ROMK2 4
    • Maintaining renal and systemic homeostasis 1
    • Possibly having immunosuppressive properties (particularly the form known as uromodulin) 5

Molecular Basis

  • Tamm-Horsfall protein is anchored to the cytoplasmic membrane via a phosphatidyl-inositol group
  • It is secreted in urine after cleavage by a specific phospholipase 5
  • The protein has a high half-cystine content (1 per 11-12 amino acid residues) 6
  • No free thiol groups are detected in the glycoprotein 6
  • The carbohydrate-protein linkages are primarily not of the O-glycosidic type 6

Understanding the Sda antigen and its biochemical basis as Tamm-Horsfall glycoprotein is important for transfusion medicine specialists when encountering patients with antibodies to this high-prevalence antigen, as these can cause clinically significant transfusion reactions requiring specialized management.

References

1
Guideline

Guideline Directed Topic Overview

Dr.Oracle Medical Advisory Board & Editors, 2025

5
Research

[Tamm-Horsfall protein].

Nephrologie, 1992

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This information is intended for healthcare professionals. Any medical decision-making should rely on clinical judgment and independently verified information. The content provided herein does not replace professional discretion and should be considered supplementary to established clinical guidelines. Healthcare providers should verify all information against primary literature and current practice standards before application in patient care. Dr.Oracle assumes no liability for clinical decisions based on this content.

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