The answer probably lies in the fact that repression of the trp operon is weak—much weaker.

TLS Online TPP Program

#Id: 7214

#Unit 5. Developmental Biology

Class II presentation can occur with either live or replication-incompetent virus; protein synthesis inhibitors had no effect, indicating that new protein synthesis is not a necessary condition of class II presentation.

TLS Online TPP Program

#Id: 7215

#Unit 5. Developmental Biology

Class II, but not class I, presentation is inhibited by treatment of the cells with an agent that blocks endocytic processing within the cell (chloroquine).

TLS Online TPP Program

#Id: 7216

#Unit 5. Developmental Biology

T cell

TLS Online TPP Program

#Id: 7217

#Unit 5. Developmental Biology

T cells initiate the adaptive immune response by interacting MHC on antigen-presenting cells that have been exposed to pathogens. 

TLS Online TPP Program

#Id: 7218

#Unit 5. Developmental Biology

Development of T cells from hematopoietic stem cells on bone marrow stromal cells expressing the Notch ligand

TLS Online TPP Program

#Id: 7219

#Unit 5. Developmental Biology

T-Cell Development

Commitment of hematopoietic precursors to the T cell lineage

The initiation of antigen receptor gene rearrangements

The selection and expansion of cells that have successfully rearranged one of their T-cell receptor genes (-selection).