Final answer:
Cellular immunity involves T cell activation and clonal expansion where T cells with specific receptors for a presented antigen are selected and proliferate, thus forming an effective immune response against pathogens.
Step-by-step explanation:
Cellular Immunity and T Cell Activation
Cellular immunity is a form of immune response that involves the activation of immune cells called T lymphocytes, or T cells. When a foreign antigen is processed and presented by antigen-presenting cells (APCs) in association with self-major histocompatibility complex (MHC) molecules, mature T cells recognize them and are activated through a process called T cell activation. This recognition, driven by the unique amino acid sequence and three-dimensional shape of the antigen-binding site on the T cell receptor, triggers T cell proliferation, known as clonal expansion.
The clonal selection process ensures that only those T cells with receptors specific to the presented antigen are selected to proliferate. As a result, a targeted pool of T cells is expanded, with each T cell being a clone of the original activated cell, sharing its specific receptor coded into the DNA. This targeted response efficiently controls the pathogen by increasing the number of specific T cells available to combat the infection.