Compositions and methods including recombinant B lymphocyte cell line including at least one endogenous gene expressing at least one endogenous membrane immunoglobulin reactive to a first antigen and including at least one exogenously incorporated nucleic acid expressing at least one exogenous secreted immunoglobulin reactive to a second antigen

1. An isolated recombinant cell line comprising:

an isolated B lymphocyte cell line including at least one endogenous gene encoding at least one endogenous membrane immunoglobulin reactive to a first antigen and at least one exogenously incorporated nucleic acid encoding at least one exogenous secreted immunoglobulin reactive to a second antigen.

2. The isolated cell line of claim 1 , wherein

the isolated B lymphocyte cell line includes at least one endogenous gene encoding at least one endogenous secreted immunoglobulin reactive to the first antigen.

3. The isolated cell line of claim 2 , wherein the first antigen and the second antigen are different epitopes of a single antigenic polypeptide.

4. The isolated cell line of claim 1 , wherein the at least one exogenously incorporated nucleic acid encoding the at least one secreted immunoglobulin reactive to the second antigen comprises at least two exogenously incorporated nucleic acid encoding the at least one secreted immunoglobulin reactive to the second antigen.

5. The isolated cell line of claim 4 , wherein the at least one exogenously incorporated nucleic acid encoding the at least one secreted immunoglobulin comprises nucleic acids encoding two heavy chain (H) immunoglobulins and two light chain (L) immunoglobulins.

6. The isolated cell line of claim 4 , wherein the at least one exogenously incorporated nucleic acid encoding the at least one secreted immunoglobulin comprises nucleic acids encoding one heavy chain (H) immunoglobulin and one light chain (L) immunoglobulin.

7. The isolated cell line of claim 4 , wherein the at least one exogenously incorporated nucleic acid encoding the at least one secreted immunoglobulin comprises nucleic acids encoding one single chain Fv immunoglobulin.

8. The isolated cell line of claim 1 , wherein the isolated B lymphocyte cell line includes at least one exogenously incorporated nucleic acid encoding at least one membrane immunoglobulin reactive to the second antigen.

9. The isolated cell line of claim 8 , wherein the at least one exogenously incorporated nucleic acid encoding at least one membrane immunoglobulin reactive to the second antigen comprises at least two exogenously incorporated nucleic acid encoding the at least one membrane immunoglobulin reactive to the second antigen.

10. The isolated cell line of claim 1 , wherein

the isolated B lymphocyte cell line includes at least one exogenously incorporated nucleic acid encoding a secreted immunoglobulin reactive to a third antigen.

11. The isolated cell line of claim 1 , wherein

the isolated B lymphocyte cell line disrupts expression of the endogenous secreted immunoglobulin reactive to the first antigen in the isolated B lymphocyte cell line.

12. The isolated cell line of claim 1 , wherein

the at least one exogenously incorporated nucleic acids encoding the at least one secreted immunoglobulin is present in one or more chromosomal loci in the isolated B lymphocyte cell line.

13. The isolated cell line of claim 12 , wherein

the at least two exogenously incorporated nucleic acids encoding the at least one secreted immunoglobulin are present in Ig H chain and Ig L chain chromosomal loci in the isolated B lymphocyte cell line.

14. The isolated cell line of claim 12 , wherein

the at least one exogenously incorporated nucleic acid encoding the at least one secreted immunoglobulin is inserted in one or more non-Ig L or non-Ig H chromosomal loci in the isolated B lymphocyte cell line.

15. The isolated cell line of claim 1 , wherein

the at least one exogenously incorporated nucleic acid encoding the at least one secreted immunoglobulin is inserted in an extrachromosomal replicating genetic element in the isolated B lymphocyte cell line.

16. The isolated cell line of claim 1 , wherein

the at least one exogenously incorporated nucleic acid encoding the at least one secreted immunoglobulin is derived from a B lymphocyte cell line.

17. The isolated cell line of claim 1 , wherein the at least one endogenous membrane immunoglobulin activated by the first antigen controls expression of the at least one exogenously incorporated nucleic acid encoding at least one secreted immunoglobulin reactive to the second antigen.

18. The isolated cell line of claim 1 , wherein

the isolated B lymphocyte cell line comprises at least one of naïve B lymphocytes, immature B lymphocytes, transitional B lymphocytes, mature B lymphocytes, follicular B lymphocytes, memory B lymphocytes, plasmablasts, or plasma cells.

19. The isolated cell line of claim 16 , wherein the isolated B lymphocyte cell line comprises a polyclonal population of B lymphocytes.

20. The isolated cell line of claim 16 , wherein the isolated B lymphocyte cell line comprises a monoclonal population of B lymphocytes.

21. The isolated cell line of claim 1 , wherein

the membrane immunoglobulin comprises at least one of a membrane anchor, a cytoplasmic domain, and an extracellular ligand-binding domain.