11. Serologic Methods and Precipitation

Question 1

What type of antigens do precipitation reactions require?

What about antibodies?

Answer 1

1. antigens must be soluble
2. antigens must have at least 2 copies of the same epitope
3. antibodies must have at least 2 available Ag-binding sites
4. Ab and Ag must be present in equivalent proportions

Question 2

Optimal antigen-antibody reactions require ________________ amounts of antigen and antibody.

Answer 2

Equivalent

Question 3

Suppose a serum sample had a very high concentration of antibody. How could this affect the patient’s result in a serology test?

Answer 3

• extreme excess of Ab result in a failure of a serological reaction to occur
• can cause a false negative result if only one dilution of serum is used
• Nephelometry = want antibody excess

Question 4

How do the size of the molecule, concentration of antigen or antibody, and temperature of reaction affect the rate of immunodiffusion?

Answer 4

1. Smaller molecules diffuse faster than larger molecules
2. The greater the concentration, the father the diffusion from source (example: the diameter around well increases)
3. Warmer temperatures speed up reactions

Question 5

In the radial immunodiffusion (RID) test, what is the concentration of antigen in the test sample proportional to?

Answer 5

The concentration of antigen in the test sample is proportional to the size of the ring

Question 6

In nephelometry, what is the amount of light scatter proportional to?

Answer 6

The amount of light scatter is proportional to the number of Ag-Ab complexes formed

Question 7

Why is nephelometry performed under conditions of antibody excess?

Answer 7

Reaction is carried out in antibody excess; otherwise, percipitation of immune complexes out of solution will occur and light scatter measurement will be falsely decreased .