Agglutination 1

Agglutination The interaction between antibody and a particulate antigen results in visible clumping called agglutination Particulate antigen include: bacteria, white blood cells, red blood cells, latex particles Antibodies that produce such reactions are called agglutinins If an agglutination reaction involves red blood cells, then it is called hemagglutination Antigens (Bacteria) RBCs Antigens

Agglutinin and Agglutinogen It is an antibody that interacts with antigen on the surface of particles such as erythrocytes, bacteria, or latex particles to cause their agglutination in an aqueous environment Agglutinogen It is an antigen on the surface of particles such as red blood cells that react with the antibody known as agglutinin to produce agglutination The most widely known agglutinogens are those of the ABO and related blood group systems

Applications Agglutination reactions now have a wide variety of applications in the detection of both antigens and antibodies including: blood grouping, diagnosis of infectious diseases measure levels of certain therapeutic drugs, hormones, and plasma proteins

Testing Abs & Ags The agglutination reaction may be used to identify the antibody or antigen in a patient sample When testing for antibody, the antigen concentration is constant for each dilution being tested When testing for antigen, the antibody concentration is constant for each dilution being tested

Agglutination & Precipitation Agglutination reactions are similar in principle to precipitation reactions; they depend on the cross linking of polyvalent antigens with the exception that: Precipitation reactions involve soluble antigens, while agglutination involves particulate antigens Pecipitation reactions represent a phase change, while the agglutination reactions manifest as clumping of antigen/ antibody complexes Agglutination is more sensitive than precipitation

Advantages of Agglutination Techniques The agglutination reaction has wide spread use in the clinical laboratory due to the following reasons: they are simple inexpensive reliable The visible manifestation of the agglutination reaction eliminates the need for complex procedures and expensive instrumentation Numerous techniques have been described for agglutination tests, these techniques may be performed using: slides, test tubes, or micotiter plates, depending on the purpose of the test However the principle of the agglutination remain the same

Qualitative and Quantitative Techniques Qualitative agglutination test Semi-quantitative agglutination test

Qualitative Agglutination Test Agglutination tests can be used in a qualitative manner to assay for the presence of an antigen or an antibody The antibody is mixed with the particulate antigen and a positive test is indicated by the agglutination of the particulate antigen For example, a patient’s red blood cells can be mixed with antibody to a blood group antigen to determine a person’s blood type In a second example, a patient’s serum is mixed with red blood cells with virus Ags to assay for the presence of antibodies to that virus in the patient’s serum

Semi-Quantitative Agglutination Test Agglutination tests can also be used to quantitate the level of antibodies to particulate antigens In this test: one makes serial dilutions of a sample to be tested for antibody and then adds a fixed number of red blood cells or bacteria or other such particulate antigen then determines the maximum dilution, which gives agglutination The maximum dilution that gives visible agglutination is called the titer The results are reported as the reciprocal of the maximal dilution that gives visible agglutination This can be done using a microtiter plate

Semi-Quantitative Agglutination Test 1/2 1/4 1/8 1/16 1/32 1/64 1/128 1/256 1/512 1/1024 Pos. Neg. Titer 64 8 512 <2 32 128 4 Patient 1 2 3 5 6 7

Steps in Agglutination Agglutination is a two-step process that results in the formation of a stable lattice network Sensitization The first reaction involves antigen-antibody combination through single antigenic determinants on the particle surface and is often called sensitization Lattice formation The second step is the formation of cross-links that form the visible aggregates This represents the stabilization of antigen–antibody complexes with the binding together of multiple antigenic determinants Each stage of the process is affected by different factors, and it is important to understand these in order to manipulate and enhance end points for such reactions

1- Sensitization Antibody molecules attach to their corresponding Antigenic site (epitope) on the particle There is no visible clumping

2- Lattice Formation Crosslinking Abs Antibody molecules crosslink the particles forming a lattice that results in visible clumping or agglutination

Factors that Affect Agglutination Buffer pH The relative concentration of Antibody and Antigen Location and concentration of Antigenic Determinants on the Particle Electrostatic Interactions between Particles Electrolyte Concentration Antibody Isotype Temperature

Hemagglutination The agglutination of red blood cells by either direct agglutination or indirect agglutination Direct agg. Ag is an intrinsic component of RBC Indirect agg. Soluble Ags are adsorbed to the RBC There are 3 ways in which Ags can be bound to RBCs: Spontaneous adsorption of Ags by RBCs Covalent binding using chemical links

The Latex particles Latex particles are usually prepared by emulsion polymerization Styrene (unsaturated liquid hydrocarbon) is mixed with a surfactant (sodium dodecyl sulfate) solution, resulting emulsified in billions of micelles extremely uniform in diameter When the polymerization is finished, polystyrene chains are arranged into the micelles with the hydrocarbon part in the center and the terminal sulfate ions on the surface of the sphere, exposed to the water phase Surfactants are usually organic compounds that are amphiphilic, meaning they contain both hydrophobic groups (their tails) and hydrophilic groups (their heads)

The Latex Particles Black ball chains represents polystyrene with sulfate free radicals (shaded balls) Blue balls denote the sulfonic acid group of the SDS Tail represent the hydrocarbon tail The simplest method of attaching proteins to the particles is by passive adsorption

Latex Agglutination In latex agglutination procedures, an antibody (or antigen) coats the surface of latex particles (sensitized latex) When a sample containing the specific antigen (or antibody) is mixed with the milky-appearing sensitized latex, it causes visible agglutination Positive Negative

Viral Hemagglutination Many viruses have nonserological hemagglutinating properties They can agglutinate RBCs in the absence of Ab (non-immune agglutination) Mammalian reoviruses agglutinate erythrocytes through interactions between the viral surface protein sigma 1 and carbohydrate groups attached to proteins on erythrocyte membranes Hemagglutination (HA) can be used to determine titers of certain viruses

Types of Agglutination Direct Agglutination Indirect or Passive Agglutination Reverse Passive Agglutination Agglutination Inhibition Coagglutination

Direct Agglutination In this reaction the antigen is an intrinsic component of the particle The agglutination test is used to determine whether antibody, specific for the antigen is present in the biological fluids serum urine or CSF Direct agglutination tests are used primarily for diagnosis of infectious diseases

Passive or Indirect Agglutination Employs particles that are coated with antigens not normally found on their surfaces Antigen has been affixed or adsorbed to the particle surface A variety of particles, including erythrocytes, latex and others are used for this purpose The use of synthetic beads or particles provides the advantage of consistency, uniformity, and stability Passive agglutination tests have been used to detect antibodies to viruses such as: cytomegalovirus, rubella, varicella-zoster, and HIV-1/HIV-2

Reverse Passive Agglutination In reverse passive agglutination, antibody rather than antigen is attached to a carrier particle The antibody must still be reactive and is joined in such a manner that the active sites are facing outward This type of testing is often used to detect microbial antigens Latex particle coated with Ab (known) + serum looking for particular Ag If Ag present, then visible agglutination is observed

Reverse Passive Agglutination Numerous kits are available for rapid identification of antigens on infectious agents Such tests used for organisms that are difficult to grow or when rapid identification is required Testing of specimens for the presence of viral antigens has still not reached the sensitivity of enzyme immunoassays But for infections in which a large amount of viral antigen is present, such as rotavirus and enteric adenovirus in infants, latex agglutination tests are extremely useful

Agglutination Inhibition Agglutination inhibition reactions are based on competition between particulate and soluble antigens for limited antibody-combining sites The lack of agglutination is an indicator of a positive reaction The technique is called hemagglutination inhibition if the particle in the reaction is a RBC

Agglutination Inhibition - Positive Tube containing free known Ab specific for the Ag to be detected Patient has Ag and will combine with Ab No visible agglutination Latex beads coated with same Ag to be detected is added It has nothing to attach to No visible reaction Therefore agglutination inhibition is positive

Agglutination inhibition - Negative Tube containing free known Ab Patient serum does not contain Ag therefore no combination Latex beads coated with same Ag to be detected is added Visible agglutination, Therefore agglutination inhibition is negative

Hemagglutination Inhibition Antibodies to the virus in the patient serum bind to the virus; blocks binding sites on the viral surfaces prevents the virus from agglutinating the red cells Example detecting antibodies to influenza Positive Negative Picture public domain Hemagglutination inhibition for detection of influenza antibodies

Coagglutination The name given to systems using bacteria as the inert particles to which antibody is attached Staphylococcus aureus is most frequently used, because it has a protein on its outer surface, called protein A which naturally adsorbs the FC portion of antibody molecules The Fab region is free to interact with antigens present in the applied specimens fragment crystallizable (FC)

hepatitis B surface antibody detection

Summary Hepatitis B surface antigen (HBsAg) is the first serologic marker, appearing in the serum 6 to 16 weeks following HBV infection In acute cases, HBsAg usually disappears 1 to 2 months after the onset of symptoms with the appearance of hepatitis B surface antibody (anti-HBs) Anti-HBs also appears as the immune response following hepatitis B vaccination

Principle When used by recommended technique, reagent will agglutinate in presence of Abs to HBV No agglutination generally indicates absence of Abs Test cells are preserved avian erythrocytes coated with Ags of HB Any non-specific reaction should be detected using the control cells Avian erythrocytes not coated with Ags of HB

Procedure for Qualitative Screening Technique Each specimen requires 3 wells of a microtiter plate To well 1 add 190 µl of diluent and 10 µl of serum, -ve or +ve controls Using a pipette, mix contents of well 1 and transfer 25 µl to wells 2 & 3 Resuspend both test and control cells and then add 75 µl of control cells to well 2 & 75 µl of test cells to well 3 Tap the plate gently to mix the contents making sure to avoid cross contamination Incubate the plate for 45-60 minutes at RT keeping the plate away from heat, direct sunlight & any source of vibration Read and record the results, the results are stable for 24 hours if the plate is covered

Interpretation of Results Test cells Control cells Strong Positive Full cell pattern covering the bottom of the well No agglutination, tight button Weak Positive Cell pattern coves 1/3 of well bottom Intermediate Cell pattern shows distinctly open center Negative Cells settled to a compact button Non-Specific Positive reaction Reference Values Hepatitis B Surface Antibody Unvaccinated: negative Vaccinated: positive