Multiple myeloma = neoplasm of plasma cells.

Plasma cells produce secretory Ab's (aka produce Ab's) -- plasma cells do not have membrane-bound Ig (aka do not have BCR').

To recap: membrane-bound Ig = BCR while secretory Ig = Ab.

Now, anti-idiotypic antibody = antibody against antibody.

Plasma cells do not contain surface Ig -- because plasma cells only be secrete Ab's.

Therefore, anti-idiotypic antibody would not work to target myeloma cells because myeloma cells = plasma cells aka plasma cells lack surface molecules that anti-idiotypic Ab's would need to bind to/target.

See diagram: https://qph.fs.quoracdn.net/main-qimg-ae42c5c9f839fc58a3ce10feaf14c02b

I forgot that Plasma cells contain SECRETORY Ig while B cells create membrane Ig

An anti-idiotypic antibody is an antibody which binds an antibody.

As an example, an anti-idiotypic antibody be helpful for treating a naiive B cell prior to activation, which express IgM and IgD on their surfaces. This is because the anti-idiopathic antibody can bind to the IgM/IgD antibody on the surface of a naiive B cell.

Multiple Myeloma cells, however, are made up of Plasma cells, which no longer express Ig's on their surface like naiive B cells do. Thus, there is no surface antibody [aka "membrane bound surface immunoglobulin"] for an anti-idiotypic antibody to bind to.

Idiotypic means --- antibody against antibody. B cells don't have surface antibodies but mere synthesize them.

Credits - Reddit : @Colden_Haulfield 7 months ago D. Multiple myeloma is due to plasma cell proliferation, which only have secretory antibodies and do not have membrane bound antibodies. An anti-idiotypic antibody would only bind the secreted antibodies by the plasma cells, but would not target the cell itself. Need to target the cell to kill it.