The Antigen-Antibody Reaction

Question 1

What is affinity?

Answer 1

the initial force that exists between an antibody Fab site and one epitope on the corresponding antigen

Question 2

What are Van der Waals forces?

Answer 2

interaction between the electron clouds of oscillating dipoles (temporary shifting of the electron clouds); dependent on how easily electron distribution can be distorted by an adjacent molecule

Question 3

What is coulombic attraction (ionic bonding)?

Answer 3

weak attraction force between positive and negative ions which increases as the charges come closer together

Question 4

What is hydrogen bonding?

Answer 4

involves an attraction between polar molecules that have a slight charge separation, and in which the positive charge resides on a hydrogen ion

Question 5

What are hydrophobic forces?

Answer 5

hydrophobic groups in nonpolar molecules associate with one another, squeezing water out as they do so, therefore being held more firmly together; estimated to be up to 50% of the total strength of antigen-antibody bonds

Question 6

What is avidity?

Answer 6

the sum of all attractive forces between antigen and antibody; measure of overall stability of the complex, which is essential to the detection of an unknown

Question 7

What effect does the kind and concentration of electrolyte present have on precipitation or agglutination?

Answer 7

saline is commonly used, as it is capable of reducing (neutralizing) the net negative charge in solution, which reduces the repulsion of molecules

Question 8

What is the optimal pH for most serological procedures?

Answer 8

6.5-8.5

Question 9

How does temperature affect precipitation and agglutination reactions?

Answer 9

the visible stage of serologic reactions is usually accelerated as temperature rises from 0C to 20-30C; however, some antigen-antibody complexes only react at very low temperatures (cold agglutinins)

Question 10

What effect does agitation have on antigen-antibody reactions?

Answer 10

accelerates the visible state of the reaction by providing increased opportunity for contact between the antigen and antibody

Question 11

What is the Law of Mass Action?

Answer 11

free reactants are in equilibrium with bound reactants

Question 12

What does the equilibrium constant (K) represent?

Answer 12

the difference in the rates of the forward and reverse reactions; can be seen as a measure of the “goodness of fit”, and depends on the strength of the bonds within the antigen-antibody complex

Question 13

What is the equation for finding K, and what do each of the factors represent?

Answer 13

K = K1/K2 = (AgAb)/(Ag + Ab); (AgAb) is the concentration of the antigen-antibody complex in mol/L, and the individual Ag and Ab factors represent the concentrations of antigen and antibody, respectively, in mol/L

Question 14

What is the general rule of antigen-antibody complexes regarding avidity?

Answer 14

as avidity increases: 1. the tendency of the complex to dissociate decreases, and 2. the value of K2 decreases as K1 increases

Question 15

How does the value of K relate to precipitation or agglutination testing?

Answer 15

the higher the value of K, the higher the amount of antigen-antibody complex, and therefore the more visible/easily detectable the reaction is

Question 16

What is the Lattice Hypothesis of Marrack?

Answer 16

most widely favored antigen-antibody complex hypothesis today; based on assumptions that each antibody molecule must have at least two binding sites, and antigen must be multivalent; therefore, antigen-antibody aggregates consist of a lattice or framework of alternating antigen and antibody molecules

Question 17

What is the prozone phenomenon?

Answer 17

if there is an excess of antibody, there will not be enough antigen to cause precipitation, leading to false negatives

Question 18

If prozone phenomenon is suspected, how can one resolve the discrepancy?

Answer 18

dilute out patient antibody and perform the test again

Question 19

What is the zone of equivalence?

Answer 19

as the optimal proportions of antigen and antibody are reached, stable lattice structures are formed and precipitate out to their maximum; required for detectable precipitation reactions

Question 20

What is the postzone phenomenon?

Answer 20

if there is an excess of antigen, all of the antibody will be bound before a precipitate can be formed (the excess antigen does not allow for the formation of the lattice); leads to a false negative

Question 21

How is the postzone phenomenon resolved?

Answer 21

test is repeated with a new patient specimen collected about a week later; this allows for further production of the antibody in the patient (if test is still negative, it is likely the patient does not actually have the antibody)

Question 22

What is neutralization?

Answer 22

the ability of an antibody to diminish or abolish some biologic property of the antigen other than its antigenicity

Question 23

What is antigenicity?

Answer 23

the toxicity or infectivity for cells, or the enzymatic activity of a given antigen

Question 24

When would a neutralization test be performed?

Answer 24

commonly used on toxins, viruses, and enzyme or enzyme-like antigen, but it can be performed on most infectious agents