An antigen-antibody reaction, also known as an antibody-antigen interaction or antigen-antibody binding, is a specific molecular interaction between an antigen and an antibody. Antigens are molecules or molecular structures often found on the surface of pathogens like bacteria, viruses, and other foreign substances, as well as on the surface of cells in the body. Antibodies or immunoglobulins are Y-shaped proteins formed by the immune system as a response to the presence of antigens.
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General feature of antigen-antibody reactions
- The reaction is specific; an antigen combines only with its homologous antibody and vice versa. The specificity however is not absolute and cross-reactions may occur due to antigenic similarity or relatedness.
- Entire molecules react and not fragment.
- There is no denaturation of the antigen or the antibody during the reaction.
- The combination occurs at the surface, therefore it is the surface antigens that are immunologically relevant.
- The combination is firm and irreversible. The firmness of the union is influenced by the affinity and avidity of the reaction.
- Affinity refers to the intensity of attraction between the antigen and antibody molecules. It is a function of the closeness of fit between an epitope and the paratope (antigen combining region of its antibody).
- Avidity is the strength of the bond after the formation of the antigen-antibody complexes. It reflects the overall combining property of the various antibody molecules in an antiserum, possessing different affinity constants with the multiple epitopes of the antigen.
- Antigens and antibodies can combine in varying proportions, unlike chemicals with fixed valencies. Both antigens and antibodies are multivalent, antibodies are generally bivalent, though IgM molecules may have five or ten combining sites. Antigens may have valencies up to hundreds.
At high antibody concentrations, the number of antibody binding sites may greatly exceed the number of epitopes present in the antigens. As a result, most antibodies bind antigen only univalent instead of multivalently. Antibodies that bind univalent can not cross-link one antigen to another.
Prozone effects are readily diagnosed by performing the assay at a variety of antibody ( or antigen) concentrations. As one dilutes to an optimum antibody concentration, one sees higher levels of agglutination. When using polyclonal antibodies incomplete antibodies also cause a prozone effect.
The interaction between an antibody and a particulate antigen results in visible clumping called agglutination. Antibodies that produce such reactions are called agglutinins. Better agglutination takes place with IgM antibodies than with IgG antibodies. Excess of an antibody also inhibits agglutination reaction; this inhibition is called the prozone phenomenon.
- Agglutination is more sensitive than precipitation for the detection of antibodies.
- Agglutination occurs optimally when antigens and antibodies react in equivalent proportions.
The prozone phenomenon may be seen when either an antibody or an antigen is in excess. Incomplete or monovalent antibodies do not cause agglutination, though they combine with the antigen. They may act as blocking antibodies, inhibiting agglutination by the complete antibody added subsequently.
Types of agglutination
- Slide agglutination: Serotyping.
- Tube agglutination: e.g. Widal test.
- Indirect (passive agglutination): where soluble antigens are coated on vehicle particles e.g. latex particles, RBCs.
- When a drop of the appropriate antiserum is added to a smooth, uniform suspension of a particulate antigen in a drop of saline on a slide or a tile, agglutination takes place.
- A positive result is indicated by the clumping together of the particles and the clearing of the drop. Depending upon the titer of the serum, agglutination may occur instantly or within seconds.
- Clumping occurring after a minute may be due to the drying of the fluid and should be disregarded.
- It is essential to have on the same slide a control consisting of the antigen suspension in saline, without the antiserum, to ensure that the antigen is not autoagglutinable.
- Slide agglutination is a routine procedure for the identification of many bacterial isolates from clinical specimens. It is also the method used for blood grouping and cross-matching.
This is the standard quantitative method for the measurement of antibodies.
When a fixed volume of a particulate antigen suspension is added to an equal volume of serial dilutions of an antiserum in test tubes, the agglutination titer of the serum can be estimated.
Tube agglutination is routinely employed for the serological diagnosis of typhoid, brucellosis, and typhus fever (weil- felix reaction).
The procedure involves adding a suspension of dead typhoid bacterial cells to a series of tubes containing the patient’s serum, which has been diluted out to various concentrations. After the tubes have been incubated for 30 minutes at 37° C, they are centrifuged and examined to note the amount of agglutination that has occurred. The reciprocal of the highest dilution at which agglutination is seen is designated as the antibody titer of the patient’s serum. For example, if the highest dilution at which agglutination occurs is 1:320, the titer is 320 antibody units per milliliter of serum. Naturally, the higher the titer, the greater the antibody response of the individual to the disease. Two types of antigens are used, the H or the flagellar antigen and the O or the somatic antigen of the typhoid bacillus. Find details information about WIDAL TEST here
Round bottomed Felix tubes are used for agglutination. Agglutinated bacilli spread out in a disc-like pattern at the bottom of the tubes. The tube agglutination test for brucellosis may be complicated by the prozone phenomenon and the presence of blocking antibodies. Several dilutions of the serum should be tested to prevent false-negative results due to prozone.
The weil- felix reaction
Weil-felix reaction for serodiagnosis of typhus fever is a heterophile agglutination test and is based on the sharing of a common antigen between typhus rickettsiae and some strains of proteus bacilli. Another example of the heterophile agglutination test is the streptococcus MG agglutination test for the diagnosis of primary atypical pneumonia. Examples of agglutination tests using red blood cells as antigens are the Paul Bunnel test and the cold agglutination test. The cold agglutination test is positive in primary atypical pneumonia. The patient’s sera agglutinate human O group erythrocytes at 4°C the agglutination being irreversible at 37°C.
Agglutination reactions are routinely performed to type red blood cells. In typing for the ABO antigens, RBCs are mixed on a slide with antisera to the A or B blood group antigens. If the antigen is present on the cells, they agglutinate, forming a visible clump on the slide. Determination of which antigens are present in donor and recipient blood is the basis for matching blood types for transfusions.
Numerous procedures have been developed to detect antigens via the agglutination (clumping) of an artificial carrier particle such as a latex bead with an antibody bound to its surface.
Antibody molecules can be bound in random alignment to the surface of latex (polystyrene) beads. Antigen present in a specimen being tested binds to the combining sites of the antibody exposed on the surfaces of the latex beads, forming cross-linked aggregates of latex beads and antigen.
- The size of the latex bead (0.8µm or larger) enhances the ease with which the agglutination reaction is recognized.
- Levels of bacterial polysaccharides detected by latex agglutination have been shown to be as low as 1.0 ng /ml because the pH, osmolarity, and ionic concentration of the solution influence the amount of binding that occurs, conditions under which latex agglutination procedures are carried out must be carefully standardized.
- Additionally, some constituents of body fluids such as rheumatoid factor, have been found to cause false-positive reactions in the latex agglutination systems available. To counteract this problem. It is recommended that all specimens be treated by boiling or with ethylenediaminetetraacetic acid (EDTA) before testing.
- Commercial test systems are usually performed on cardboard cards or glass slides; manufacturers’ recommendations should be followed precisely to ensure accurate results.
- Reactions are graded on a 1+ to 4+ scale, with 2+ usually the minimum amount of agglutination seen in a positive sample.
- Control latex (coated with antibodies from the same animal species from which the specific antibody was made) is tested alongside the latex. If the patient specimen or the culture isolate reacts with both the test and control latex, the test is considered nonspecific and therefore uninterpretable.
- Latex tests are very popular in clinical laboratories to detect antigens to Cryptococcus neoformans in CSF or serum and to confirm the presence of beta-hemolytic streptococcus from the culture plates. Latex tests are also available to detect Streptococcus agalactiae, Clostridium difficile toxins A and B, and rotavirus.
In this case, the particles are killed and treated Staphylococcus aureus (Cowan I strain), which contains a large amount of an antibody-binding protein, protein A, in their cell walls. In contrast to latex particles, these staphylococci bind only the base of the heavy chain portion (Fc) of the antibody, leaving both (Fab) antigen-binding ends free to form complexes with specific antigens. Several commercial suppliers have prepared coagglutination reagents for the identification of streptococci, including Lancefield groups A, B, C, D, F, G, and N; Streptococcus pneumoniae; Neisseria meningitidis; N gonorrhoeae; and Haemophilus influenzae types A to F grown in culture. The coagglutination reaction is highly specific but may not be as sensitive for detecting small quantities of antigen as latex agglutination. Thus, it is not usually used for direct antigen detection.
Latex agglutination inhibition test
The latex agglutination inhibition test relies on the competition for the antibody between a latex-drug conjugate and any drug that may be present in the sample (mostly urine). A urine sample is placed in the mixing well of a slide containing antibody reagent, buffer, and latex reagent. a) If the drug is absent, the latex-drug conjugate binds to the antibody and forms large particles that agglutinate. Therefore agglutination is evidence for the absence of drugs in the urine specimen b) If a drug is present in the urine sample, it competes with the latex conjugate for the small amount of available antibody. A sufficient quantity of the drug will prevent the formation of particles and agglutination and a positive urine sample does not change the appearance of the test mixture.
- Direct Coombs test: Detection of incomplete antibodies on patients’ RBCs. Antibodies attached on the surface of the RBCs ( patient RBCs) + Antihuman globulin = agglutination.
- Indirect Coombs test: Detection of antibodies in patients sera. Rhesus positive RBC + Patient serum ( if contains incomplete circulating Abs coats the surface of the RBC)+ Antihuman globulin which makes the bridge = agglutination