sensitization and agglutination
DESCRIPTION
Sensitization and Agglutination. Antigen-Antibody reaction. Red cell Ag-Ab reaction can be 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 - PowerPoint PPT PresentationTRANSCRIPT
Sensitization and Agglutination
Antigen-Antibody reaction
• Red cell Ag-Ab reaction can be 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• In vitro, lysis of RBC’s rarely occurs.
– Agglutination
• Used as indicator of Ag-Ab reaction
Sensitization
• Sensitization is whereby the Ab binds to Ag on the surface of the cell.
• Sensitization does not always result in visible agglutination.
• IgG is too small to span the distance between two red cells.
• So IgG does not always result in visible agglutination.
Agglutination
• Agglutination occurs when bound Ab links
adacent red cells forming clumps.
• Red cells must be close enough for the
Fab portion of Ab to bind and make
bridges between cells.
• IgM can easily induce agglutination.
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.
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
Stage 1: Sensitization
This represents what occurs during stage 2 of agglutination:
Antibody molecules crosslink RBCs forming a lattice that results in visible clumping or agglutination.
Stage 2: Lattice Formation
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
• 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
Factors affecting the Zeta Potential
1- Using Enzyme
2. Introduction of bipolar Albumin
• Albumin dissipates some of the +ve charges around cells, reducing zeta potential
Albumin
Factors affecting the Zeta Potential
3. 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
Increase the ionic strength of the medium
Factors affecting the Zeta Potential
4. 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
Decreasing the ionic strength of medium by using low ionic strength saline (LISS)
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
Factors affecting Red Cell Sensitization
2- The pH of reaction mixture– At a pH below the
isoelectric point, Abs have +ve charges
– This makes it easier for the Ab to bind to the –vely charged red cells
– Optimal pH for sensitization is 6.5 to 7.5 (Ab +vely charged)
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
• 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
4oC 37oC
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
Factors Influencing RBCs Agglutination
• Agglutination occurs when RBCs are close enough allowing the Ab to bridge adjacent cells
Factors Influencing RBCs Agglutination
1- Ionic strength of Medium– Rate of sensitization increases in LISS
BUT:– Agglutination is impaired because of increase
in zeta potential
• (Rule: reduced zeta potential facilitate agglutination)
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 (previously discussed)
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
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
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
IgMAg
300 Ao
IgG IgG
150 Ao250-300 Ao
7- Antibody Characteristics
Summary
Sens. Agg.
Removal of RBCs sialic acid by enzymes
Introduction of bipolar albumin ___
Increasing ionic strength of the medium
decreasing ionic strength of the medium