IgA is the principal class in secretions, the most important being those from the epithelia lining the intestinal and respiratory tracts.

Dimeric IgA operates mainly on epithelial surfaces.

This monomeric IgA is predominantly of the subclass IgA1; the ratio of IgA1 to IgA2 in the blood is 10:1.

IgA antibodies produced by plasma cells in the gut are dimeric and predominantly of subclass IgA2; the ratio of IgA2 to IgA1 in the gut is 3:2.

#Id: 2512

#Unit 13. Methods in Biology

Quantitative analysis (i.e., measurements of the relative amounts of different proteins in a sample) can be achieved by scanning densitometry .

#Id: 2513

#Unit 13. Methods in Biology

Larger fragment DNA are isolated by using agarose gel while shorter fragment are separated by 3.5% polyacrylamide gels are used to separate DNA in the range 80–1000 nt and 12% gels to resolve fragments of between 20 and 100 nt.

#Id: 2514

#Unit 13. Methods in Biology

Pulsed-Field Gel Electrophoresis are used to separate very large fragment of DNA 2 × 103 kb, Essentially, this allows the separation of whole chromosomes by electrophoresis.

#Id: 2515

#Unit 13. Methods in Biology

Pulsed-Field Gel Electrophoresis involves electrophoresis in agarose, where two electric fi elds are applied alternately at different angles for defined time periods (e.g. 60 s)

#Id: 2516

#Unit 13. Methods in Biology

 PFGE has proved particularly useful in identifying the course of outbreaks of bacterial food-borne illness (e.g. Salmonella infections).

#Id: 2517

#Unit 13. Methods in Biology

DNA from Salmonella species, when digested with the restriction enzyme Xba I, gives around 15 fragments ranging from 25 kb to 680kb. This pattern of fragments, or ‘fingerprint’, is unique to that strain. If the same fingerprint is found from bacteria from other infected people, then it can be assumed that they were all infected from a common source which can be analyzed by PFGE