Final answer:
The antigen challenge and clonal selection primarily occur in the lymph nodes for T cells and involve the immune system selecting and proliferating only those T or B cells with receptors specific to the foreign antigen. This leads to both a vigorous immune response to fight the pathogen and the creation of memory cells for ongoing immunity.
Step-by-step explanation:
Antigen Challenge and Clonal Selection
The antigen challenge and clonal selection occur within the immune system when a specific antigen is first encountered. In T cells, this process usually takes place in the lymph nodes, where mature T cells become activated by recognizing processed foreign antigens in association with a self-major histocompatibility complex (MHC) molecule. This interaction leads to rapid cell division, known as clonal expansion, ensuring that the immune response is powerful enough to effectively control a pathogen.
Clonal selection is the process by which only T cells (or B cells) with receptors specific to the antigen in question are selected to proliferate. Naïve T cells, which are T cells that have not yet encountered their specific antigen, become activated upon antigen binding. After activation, these selected clones proliferate and form many copies of each cell, with each clone possessing a unique receptor specific to the antigen. This results in a polyclonal response involving multiple T cell clones.
Similarly, for B cells, clonal selection also occurs after recognition of the antigen. Only B cells with receptors that match the antigen are selected for proliferation. This process eventually leads to the generation of plasma cells, which secrete antibodies with the same antigenic specificity as the B cell receptors that first recognized the antigen, alongside memory B cells, which serve as a long-term immune defense.