Immunohematology Theory and Serology
Core framework
Questions here test whether you can separate what the antibody is from what the phase says. Know ABO genetics (H, A, B, Se alleles), Landsteiner's rule, and the immunoglobulin correlates: ABO antibodies are IgM, naturally occurring, and react at room temperature (immediate spin, reverse grouping), whereas immune alloantibodies (Rh, Kell, Duffy, Kidd, MNS) are IgG and need 37 °C/AHG or a gel column to demonstrate.
Concepts to master
- Agglutination grading, and the classic false positives: rouleaux, fibrin strands, Wharton's jelly, and contaminated samples.
- Direct vs indirect antiglobulin testing; a positive DAT means in vivo coating, not an antibody in the serum.
- Enzyme treatment (ficin/papain): enhances Rh, Kidd, Lewis, P1; destroys Duffy, MNS and Xg-a.
- Neutralization with soluble blood group substance; adsorption and elution to dissect mixtures.
- Dosage-prone antigens (M/N, S/s, Jk-a/Jk-b, Lu-a, Do) require homozygous cells for confident exclusion.
Traps
The most common trap is the antibody you failed to rule out. A specificity may be excluded only when at least two (ideally three) antigen-positive panel cells are nonreactive - and for dosage-prone antigens those cells must be homozygous-positive. A positive autocontrol does not exclude an alloantibody; it tells you an adsorption is needed. Also watch for cold autoantibodies masking an underlying IgG alloantibody.
Worked example
A serum is nonreactive with two Jk(a+b+) screening cells but reacts at AHG with a Jk(a+b−) panel cell. Because Jk-a shows dosage, heterozygous cells can miss the antibody. The correct interpretation is probable anti-Jk-a, not a negative screen; confirm with Jk(a+b−) cells and a sensitive method (gel or PEG-AHG).