Antibody-Antigen Interaction

4 sources involved in the interaction between an antibody and an antigen

1. Electrostatic interactions between charged side chains

2. Hydrogen bonds

3. Van der Waals forces

4. Hydrophobic interactions

3 types of antigen-binding sites

1. Crevice (pocket)

2. Groove

3. Extended surface site

Cross-reactivity or multispecificity

Occurs when an antibody occasionally binds more than one antigen because a sufficient number of chemical interactions exist between the antigen and the antibody to create a stable structure , regardless of the total “goodness of fit”.

ELISA (Enzime-linked immynosorbent assay)

It is a very sensitive test that uses a covalent complex of an enzyme linked to an antibody either to detect antigen directly or to bind to an antibody-antigen complex.

1. Antigens are bound to the bottom of each well.

2. Serum is added: a. 1 well: from a donor who has antibodies as a result of vaccination. This is the positive control. b. 2nd well: from a patient who hasn’t been vaccinated or had infection and doesn't have the antibody. Any of the relevant antibody will bind to antigen. Irrelevant antibody should be washed off.

3. Anti-human immunoglobulin with HRP (horseradish peroxidase) added to the wells and they bind to antibody which are still present in the wells.

4. The result: positive control - a filled colored product; negative control - remains empty

Lateral flow test

Is a simple test to determine whether a protein, antigen, or antibody is present in a body fluid.

Pregnancy test

COVID test

Immunofluorescence

Is used to detect antigens in cells or tissue sections. And to screen for antibodies to cell or tissue antigens.

Direct: antibodies directly linked to the fluorescent compound. We add the serum to these antibodies, they find antigens —> fluorescence.

Indirect: a ligand that can identify the antibody is linked to the fluorescent compound (often the fluorescent ligand is a second antibody that is specific for the test antibody).

Flow cytometey

It's a technique used to enumerate cells that express an antigen. The cells are stained with an antibody specific for the cell-surface or intracellular antigen. The antibody is coupled to specific fluorescent reagents, and is then passed the flow cytometer.

The # of stained cells can be counted.