Why are antibodies based on a four-chain structure, consisting of two identical heavy chains and two identical light chains, and not double-chained or six-chained? And why do Fab arms consist of three complementarity determining regions (CDRs), not two or four? How to explain this phenomenon using Darwin's evolutionary theory?
My thinking:
The triangle is the most stable and economic structure.
Two Fab arms are enough for cross linkage with more than one antigen and benefit for coagulation and recognition.
well, there are other variants of antibodies around: check the camelides single-chain antibodies, or the shark ones...
and do not forget that antibodies are just a member of a wider family of molecules based on the same Ig-like basic domain...
Thank you for your information! I just checked: Sharks may be the most primitive animals owning antibodies, which got only two CDRs with different sizes. Guided by Occam's razor, now that single-chain or two-CDRs antibodies are useful, why should we human and other animals be more complex?
I wanted to talk about Igs, but I didn‘t want to make it more complicated. However, if one CDR is big and specific enough to grab antigen, there seems no need to develop six CDRs.
Just like that familiar question "Why do people have five fingers", I'm asking why we have a pair of claws consisting of 3 CDRs. There must be some reason for the evolution.
maybe look on www.chromotek.com There you can find the story about NanoTraps. Small antibodies derived from lamas.
Hi, I think in the processes like "Opsonisation" and "ADCC" in which immunoglobulin has a central role, two branch that must be adhered to a cell, is better than one because of better and stronger adhesion they would cause. Suppose when a vehicle attached to a constant building, in this case, like above, it can be more steadier.
About CDRs, when we have six instead of one (the third CDR in each domain is variable as compared with two others), it can be seemed that a various collection of antigens can be identified. In fact, the power of recognition of immunoglobulin could be promoted. In other word, there is no any antigen in the nature which can not be detected by our immune system (Humoral immunity).
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