Sensitization and Agglutination

1. Sensitization and Agglutination M. Zaharna Blood Bank Lab. 2009

2. Antigen-Antibody reaction • Red cell Ag-Ab reaction can detected by a number of techniques • Most frequently used • Hemolysis • Occurs if the entire complement sequence is activated following Ag-Ab interaction • Frequently stops at C3 – no lysis • Agglutination • Used as indicator of Ag-Ab reaction M. Zaharna Blood Bank Lab. 2009

3. Agglutination Reactions • Two Stage Process: • Stage 1 Sensitization: • attachment of Antibody to Antigen on the RBC membrane. • Stage 2 Lattice formation (agglutination): • formation of bridges between the sensitized red cells to form the lattice that constitutes agglutination. M. Zaharna Blood Bank Lab. 2009

4. Stage 1: Sensitization This represents what occurs during stage one of agglutination. • Antibody molecules attach to their corresponding antigenic site (epitope) on the red blood cell membrane. • There is no visible clumping. • Red cells must be close enough for the Fab portion of Ab to bind and make bridges between cells M. Zaharna Blood Bank Lab. 2009

5. Stage 2: Lattice Formation This represents what occurs during stage 2 of agglutination: Antibody molecules crosslink RBCs forming a lattice that results in visible clumping or agglutination. M. Zaharna Blood Bank Lab. 2009

6. Sensitization by IgG does not result in agglutination • IgG is too small to span the distance between two red cells • IgM can easily cause agglutination • For agglutination to occur, the repulsive forces keeping red cells apart must be overcome M. Zaharna Blood Bank Lab. 2009

7. M. Zaharna Blood Bank Lab. 2009

8. The Zeta Potential • The electric repulsion between cells • This explains why cells do not agglutinate • Red cells have negative charge due to sialic acid molecules • When red cells are in solution containing free ions: • Cations are attracted to the –vely charged red cells • This forms a repelling cloud around the cell M. Zaharna Blood Bank Lab. 2009

9. The Zeta Potential can be varied by altering the charge on red cells • This can affect both sensitization and agglutination • Reducing the cloud density allow Abs to approach the cells, sensitize and then agglutinate them M. Zaharna Blood Bank Lab. 2009

10. Factors affecting the Zeta Potential • Removal of sialic acids by enzymes • Introduction of bipolar Albumin • Albumin dissipates some of the +ve charges around cells, reducing zeta potential Albumin M. Zaharna Blood Bank Lab. 2009

11. Factors affecting the Zeta Potential • Increase the ionic strength of the medium • Increasing conc. of cations in medium cause • Increase in the density of ions around the red cell which cause • Size of cloud of cations is decreased • Zeta potential decreases • Red cell approach each other easily • Agglutination is facilitated M. Zaharna Blood Bank Lab. 2009

12. Factors affecting the Zeta Potential • Decreasing the ionic strength of medium by using low ionic strength saline (LISS) • Decreasing conc. of cations in medium • Leads to decrease in density of ions around red cells • This increases sensitization • But decreases agglutination M. Zaharna Blood Bank Lab. 2009

13. Factors affecting Red Cell Sensitization 1- Ratio of Ab to Ag • Sensitization occurs easily when at higher conc. of Ab • This can be done by increasing conc. of serum containing the Ab to conc. of cells M. Zaharna Blood Bank Lab. 2009

14. Factors affecting Red Cell Sensitization 2-The pH of reaction mixture • At a pH below the pI, Abs have +ve charges • This makes it easier for the Ab to bind to the –vely charged red cells • Optional pH for sensitization is 6.5 to 7.5 (Ab +vely charged) pH 8 pH 7 M. Zaharna Blood Bank Lab. 2009

15. Factors affecting Red Cell Sensitization 3- Temperature • Ag- Ab reactions are exothermic • Therefore, Abs bind to a greater degree at lower temperature • But at lower temperatures, rate of reaction is reduced • To speed up reaction, tests are done at 37oC M. Zaharna Blood Bank Lab. 2009

16. Temperature can also affect Ag accessibility on red cells • Some IgM Abs bind best at 4oC (cold Abs) • Temperature can make conformational changes in the Ag • More Ag sites are exposed as the temperature is lowered allowing increased binding of Ab • Most naturally occurring cold Abs are of no clinical significance • Compatibility testing is done at 37oC 37oC 4oC M. Zaharna Blood Bank Lab. 2009

17. Factors affecting Red Cell Sensitization 4- Ionic strength of the medium • When RBCs are suspended in LISS the cloud of ions around the cell is less dense than in isotonic saline • Reduced conc. of cations surrounding RBCs allow +vely charged Abs easier to access Ag sites • Rate of sensitization increases M. Zaharna Blood Bank Lab. 2009

18. Factors Influencing RBCs Agglutination • Agglutination occurs when RBCs are close enough allowing the Ab to bridge adjacent cells 1- Ionic strength of Medium • Rate of sensitization increases in LISS • Agglutination impaired because of increase in zeta potential M. Zaharna Blood Bank Lab. 2009

19. Factors Influencing RBCs Agglutination 2-Presence of Albumin in medium 3- Enzyme treatment of red cells • Remove –vely charged sialic acid • & therefore reduce the zeta potential • This make cells come closer & agglutinate • But, certain Ags can be destroyed by enzymes (M, N, S, Fya, Fyb) 4- Temperature M. Zaharna Blood Bank Lab. 2009

20. Factors Influencing RBCs Agglutination 5- Antigen Density • The greater the number of Ags on red cell, the greater the sensitization • Binding of +vely charged Abs to red cells lower the zeta potential • And therefore enhances agglutination • Increased Ag density also increases chance of bridging M. Zaharna Blood Bank Lab. 2009

21. Factors Influencing RBCs Agglutination 6- Ag Clustering and Mobility • Clustering facilitates agglutination by increasing likelihood of Ab binding at that site • Cluster of some Ags can occur after enzyme treatment of cells Clustering of Ags M. Zaharna Blood Bank Lab. 2009

22. IgM Ag IgG Factors Influencing RBCs Agglutination 7- Antibody Characteristics • Ability of Ab to agglutinate cells depend on the Ig class • IgM has a wider span than IgG, and therefore more effective agglutination • IgG can be chemically modified to increase its span 300 Ao 250-300 Ao 150 Ao IgG M. Zaharna Blood Bank Lab. 2009

23. Summary M. Zaharna Blood Bank Lab. 2009