Diagnosis based on antigen and antibody reaction is known as Immunodiagnosis. Many immunological methods have been developed to diagnose not only pathogenic infection but other conditions of human body like determination of blood groups. Today immunodiagnosis is more popular in determining the quantity of normal and abnormal proteins in human body. The immunodiagnosis offers improved testing technologies where diagnosis could be faster, cheaper and earlier. The most important immunological methods include;

  • Venereal Disease Research Laboratory Test
  • Widal Test
  • Blood Grouping
  • Rh Typing
  • Immuno Electrophoresis
  • Enzyme Linked Immuno Sorbent Assay
  • Radio Immuno Assay

Venereal Disease Research Laboratory Test (VDRL)

This test is employed to detect infection of Syphilis, a sexually transmitted disease. Syphilis is a spirochaete bacterium, Treponema pallidum. The infection leads to the production of antibodies in the blood. The blood serum of the patient is taken and spread on a glass after which the VDRL antigen is applied. If the patient is actually having Syphilis, the added VDRL antigen is agglutinated by the antibodies in the serum. This can be observed by the formation of floccules on the glass slide.

Widal Test

This test is specifically used to determine the typhoid infection. Typhoid is caused by the bacterium Salmonella Typhi. The bacterium on its surface has two antigens, O and H. The patient`s serum is applied on glass slide and is tested for the presence of antibodies against the both antigens separately. If the patient really has typhoid, precipitation can be observed on glass slide.

Blood Grouping

The most important blood grouping in humans is ABO system, developed by Karl Landsteiner. Humans can be divided into four groups based on the presence and absence of `S` and `B` antigens on the surface of red blood cells (RBC). Presence of `A` antigens on RBC indicates `A` group and presence of `B` antigen indicates `B` group. The presence of A and B antigens on RBC indicates AB group while the absence of both A and B indicates O group.

In the test the person`s blood smear has to be prepared on two glass slides. Later on one slide, anti – A antibodies and on the other slide anti-B antibodies should be added. If agglutination is observed on both slides, the person belongs to `A` group, but if the agglutination is found in only first case and no agglutination in second case, then the person belong to `OA` group. If the agglutination is found only on second slide then person belongs to `B` group. If agglutination is not found on both slides, then the person belongs to `O` group.

Rh Typing

Landsteiner and Weiner found a new antigen on human RBC called Rh antigen (Rh stand for `Rhesus`). Irrespective of ABO system a person`s RBC may or may not have `Rh` antigen. To identify whether a person is Rh positive or Rh negative, first person`s blood should be spread on a glass slide. Later anti - Rh antibodies are applied. If the person has Rh antigen, agglutination can be seen on the glass slide. If the person has no Rh antigen, then no agglutination can be observed.

Immuno Electrophoresis

It is an immunological method used to separate and identify a particular antigen from different serum proteins. In this method serum proteins are first separated by electrophoresis on an agar plate on the same plate, antibodies specific to the desired antigen are introduced. Diffusion of these antibodies takes them to desires antigen on the plate. If the antigen really occurs, the antigens – antibody reaction is visible in the form of precipitation.

This technique is used to detect the presence of both normal and abnormal proteins in urine, blood and body fluids. In particular, this method is used to detect normal and abnormal serum proteins. It is also used in detecting myeloma proteins.

Enzyme Linked Immuno Sorbent Assay (ELISA)

This is an immunological technique used for quantitative determination of both antigens and antibodies. This was first developed by Engvall and Perlmann in 1970. There are two types of ELISA; Direst Elisa to identify antigen, Indirect Elisa to identify antibody.

Indirect Elisa is most successful method for detecting HIV infection. ELISA basically involves detection and estimation of either antigen or antibody through the mediation of enzyme linked antibodies that turn a substrate into a chromogenic product. Many human proteins both normal and abnormal can be detected and quantified by ELISA.

Radio Immuno Assay (RIA)

It is the most sensitive immunodiagnostic test available today. This method facilitates identification of desired antigen or any human hormone or other protein even present in the order of pictograms in the given serum or any other sample. This technique is based on the competition between the desired unlabeled antigen and radio labeled antigen to bind to an antibody. Thus it helps in identifying the desired unlabeled substance.

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