Antibody fragments for in vivo studies: Fab and F(ab')₂
Antibody format is an experimental variable, not a passive one. A full-length antibody does more than bind its target, and sometimes that extra activity is exactly what you do not want in a given experiment. Choosing a fragment is a way of deciding, deliberately, what your antibody is allowed to do. Use this page to match the format to your experimental question.
What Fab and F(ab')₂ are
A full antibody has two functional regions: the two arms (the Fab) bind the target, and the tail (the Fc) engages the immune system through Fcγ receptors and complement. From a full antibody you can derive two fragments.
| Format | Structure | Fc | Behavior |
|---|---|---|---|
| Full IgG | Two arms, bivalent | Present | Binds through both arms; Fc engages Fc receptors and complement, which can drive depletion or activation |
| Fab (~50 kDa) | One arm, monovalent | Absent | No Fc-receptor or complement engagement; one arm cannot crosslink the target, so no clustering-driven activation |
| F(ab')₂ (~110 kDa) | Two arms, bivalent | Absent | No Fc, same as Fab; two arms retain crosslinking and avidity, useful when bivalent binding matters |
Two independent levers: Fc and valency
Changing format moves two levers, and they are independent. The Fc can be present or removed: remove it and there is no Fcγ-receptor or complement engagement, so that immune activity no longer contributes to the readout. Valency can be one arm or two: two arms can crosslink and cluster the target and add avidity, one arm cannot. Crosslinking is driven by arm count, not the Fc, the F(ab‘)₂ shows this, with no Fc yet still able to crosslink. Because the levers move independently on the same clone, changing one at a time means any shift in the result traces to the lever you moved, not to a different antibody.
The matched-format control set
The cleanest way to attribute an effect is to hold the clone constant and vary only the format, so each comparison isolates one variable.
Same clone throughout, so an effect traces to the Fc or to valency, not to a difference between two binders.
When a fragment is the right tool
- Block or occupy a receptor or checkpoint in vivo without triggering FcγR- or complement-mediated depletion.
- Separate target-binding effects from Fc-dependent effects by comparing formats ofactivity, no crosslinking, target binding only. the same clone.
- Avoid antibody-mediated crosslinking or receptor clustering with a monovalent Fab, or retain bivalent binding without the Fc with an F(ab’)₂.
Why Bio X Cell for fragments
- Isolate binding from Fc effects: no Fc-mediated activity, no crosslinking, target binding only.
- Compare the same clone across formats: available as full-length, Fc-silenced, and Fab to help separate Fc-driven effects from binding-driven effects.
- Trust clones researchers already know: clones with an established citation record, available as Fab fragments from milligram to gram scale. Low endotoxin and carrier protein-free.
Fragments in practice
In a published in vivo study, an anti-CD3 F(ab‘)₂ (145-2C11) was used with its matched F(ab‘)₂ isotype control as one deliberately chosen, correctly controlled piece of a larger mechanistic design.
Read the summaryCatalog depth
785+ catalog antibodies · ~30,000 product citations in peer-reviewed publications · 12 research areas · the original in vivo antibody platform since 1997. Almost any antibody in the catalog can be made to your target as a fragment.
Ready-to-order fragments
Flagship depletion and checkpoint clones, ready to order. A matched isotype control (hamster IgG F(ab‘)₂, BE0091-FAB) is available.
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