Monovalent v. Bivalent Antibodies

[justadream]

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Is the distinction that:
Monovalent: Bind one antigen
Bivalent: Bind two antigens

OR:
Monovalent: Bind one epitope
Bivalent: Bind two epitopes


If is it the latter, then doesn't that mean that bivalent antibodies can theoretically bind twice to the same antigen (if the antigen has 2 epitopes that bind to the bivalent antibody)?

 

[type12]

Is the distinction that:
Monovalent: Bind one antigen
Bivalent: Bind two antigens

OR:
Monovalent: Bind one epitope
Bivalent: Bind two epitopes


If is it the latter, then doesn't that mean that bivalent antibodies can theoretically bind twice to the same antigen (if the antigen has 2 epitopes that bind to the bivalent antibody)?

I think the issue is definitional: what do you think an antigen is vs what do you think an epitope is?

 

[justadream]

@type12

Antigen = some overall bigger thing the antibody will bind to (e.g., a bacteria cell)
Epitope = the actual thing on the bacteria that the antibody binds to (e.g., the exact protein on the cell wall)

 

[type12]

@type12

Antigen = some overall bigger thing the antibody will bind to (e.g., a bacteria cell)
Epitope = the actual thing on the bacteria that the antibody binds to (e.g., the exact protein on the cell wall)

Ah, okay, it's just semantics 🙂

http://en.wikipedia.org/wiki/Antigen

In short, you're correct with the terms, but the scales can be smaller. The antigen could be a protein on the cell wall, and then there is a portion of that protein the antibody will bind to (it doesn't bind to the entire protein, especially if it is large), and that's referred to as an epitope. Basically, it doesn't have to be the actual invader, but just a part of it.

So, you CAN have multiple antibodies for a specific antigen, but this is argumentative. One could say that, since, there is another epitope (site on the same protein a different antibody can bind to), that portion of the protein is essentially another antigen. So, on the MCAT, you can think "an antibody can only bind to a single antigen," but it may or may not be the other way around.

You're at the point where now you're just battling semantics. Focus on the meaning/understanding (which you already have), and you'll be fine. Good question, though, and hope this clarifies!

 

[The Brown Knight]

I speak about COMMON/GENERAL cases when I say this:
one antibody binds to one epitope
many antibodies bind to one antigen

 

[justadream]

@The Brown Knight

When you say "many antibodies bind to one antigen"

Do you mean many antibodies of the same type (clones) or many different types of antibodies (non-clones)?

 

[The Brown Knight]

As far as I know, antibodies are specific to a single antigen (measles, chickenpox, etc.), so CLONES should bind to one antigen.

(A counterargument I have with myself is that somatic recombination gives B cells their diversity, and even when they are selected for immunocompetence and self-tolerance they should still be diverse enough to bind different types of epitopes - from this I would expect NON-CLONES to bind to a single antigen at different epitopes)
My answer above holds for MCAT purposes, but someone with a better immunology background can verify.