The most likely explanation for the failure of bm12 mice to make a CD4 T cell response to bovine insulin is because the bovine insulin [A 1-14] fails to bind to bm12.
This happens because MHC complexes on antigen-presenting cells will be recognized as foreign by T cells of bm12mice due to which it fails to respond against bovine insulin.
T cells are specific against a particular antigen and are manufactured in the thymus. Once released from the thymus they remain circulating in the body until they recognize their specific antigen on the APCs [antigen presenting cells]. Then CD4+ helper T cells and CD8+ cytotoxic T cells come into action and bind to the specifically recognized antigen forming an MHC complex over the surface of APCs triggering the activation of the T cells in the secondary lymphoid organs.
Learn more about T cell response mechanism here
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