Final answer:
When a foreign epitope is recognized by a lymphocyte, it triggers activation of the T cell, followed by clonal expansion for a strong immune response, and in B cells, it leads to differentiation into plasma cells that produce antibodies.
Step-by-step explanation:
What happens when a foreign epitope is recognized by the lymphocyte receptor? When a T cell encounters a foreign epitope presented in association with an MHC II molecule, it becomes activated.
The process involves the T cell recognizing a processed antigen presented by Antigen Presenting Cells (APCs) such as macrophages. These APCs, after phagocytosis and processing of the antigen, present the epitope alongside MHC molecules on their surface.
Activated T cells then undergo clonal expansion, where they rapidly divide to produce many cells all specific to the same antigen. This is crucial for a potent immune response capable of effectively combating pathogens.
B cells, meanwhile, can bind antigens via their IgM receptors, process them, and present them with MHC II to T helper cells, which leads to B cell differentiation into antibody-secreting plasma cells.