Antigen Antibody ReactionBy:

Dr. NEHAL AHMED NOUH

• Immunoassays for the detection of the antigens of microorganisms remain important tools for the diagnosis and management of infectious diseases.

• Antigen detection methods are especially useful for rapid diagnosis, (in the clinic, emergency department, or doctor’s office).

• Recently, simple one-step assays have been introduced that can provide results in 15 min.

• The basis for antigen detection assays is the specific binding of an antigen (protein or glycoprotein) to an antibody.

Antibodies are proteins synthesized and secreted by B cells that bind to antigens. Most antigens are macromolecules: proteins, polysaccharides, even DNA and RNA.

1- Agglutination:• Agglutination methods use the antibody–antigen bond to

create clumping (agglutination) of particles.

• In a typical agglutination assay for detection of microbial antigen:

1- A drop of liquid suspension of antibody-coated particles is placed on a card.

2- and the specimen is added and mixed.

3- The card is then incubated, and read by visually observing the clumping reaction.

The interaction of particulate antigens (cells that carry antigens) with antibodies leads to agglutination reactions.

Agglutination example: (Blood Grouping)

• When agglutination reactions involve the clumping of red blood cells, they are termed hemagglutinations.

• Hemagglutination occurs because of the ability of antibodies to cross-link red blood cells by binding to surface antigens.

• Hemagglutination microscopic-slide tests are used in blood typing.

• The four major human blood types (A, B, AB, and O).• Individuals with blood types A, B, or AB have antigens on the

surface of their red blood cells. • These antigens, known as agglutinogens, correspond to the

specific group (e.g., type A has agglutinogen A). • An individual having blood type O lacks these agglutinogens.

Slides Illustrating Human ABO Blood Groups After Adding Antisera A to the Blood on the Left Part of the Slide and Antisera B to the Blood on the Right Part of the Slide. The top slide shows no clumping; thus, it is blood group O. The second slide shows clumping on the right but not on the left; thus it is blood group B. The third slide showsclumping on the left but not on the right; thus it is blood group A.The bottom slide shows clumping on both left and right; it isblood group AB.

Antisera Blood groupingA B

No clumping No clumping Oclumping No clumping A

No clumping clumping Bclumping clumping AB

Blood Grouping Experiment (Amrita University) - YouTube.flv

Widal Test• The widal test is diagnostic tests for typhoid fever.• This test demonstrates the presence of somatic (O) and

flagellar (H) agglutinins to Salmonella typhi in the patient's serum using suspensions of O and H antigens.

• Antigens of S. paratyphi A, S. paratyphi B, S. paratyphi C are included in most commercial kits.

• Place a drop of O or H antigen in the circle of given white card.

• Then add one drop of serum and mix thoroughly by gentle rotation for about 1 minute.

• Formation of clumps will indicate a positive test.

2- Precipitation Reactions :• The interaction of soluble antigens with IgG or IgM

antibodies leads to precipitation reactions.

• Precipitation reactions depend on the formation of lattices and occur best when antigen and antibody are present in optimal proportions.

• Excesses of either component decrease lattice formation and subsequent precipitation.