Final answer:
B cells are activated by binding to an antigen and receiving co-stimulatory cytokines from a Th cell. This results in clonal proliferation and differentiation into memory B cells and antibody-secreting plasma cells.
Step-by-step explanation:
B cells are activated when antigen binds on the B cell surface followed by a co-stimulatory signal from a Th cell. The process of B cell activation is complex and requires two distinct signals. The first signal is the recognition of the native antigen by the B cell's surface immunoglobulin. After this initial binding, the B cell internalizes and processes the antigen, presenting fragments of it on a class II MHC molecule. The presentation attracts a helper T cell (Th2 cell), which, upon recognizing the antigen-MHC II complex, binds to the B cell using its antigen receptor. This interaction leads to the Th cell being activated and subsequently secreting cytokines that further stimulate the B cell.
The second necessary signal involves these cytokines diffusing to the B cell, which induces complete activation. At this stage, the B cell acts as an antigen-presenting cell (APC), and it begins to proliferate, producing clonal daughter cells. This clonal proliferation, known as clonal selection, increases the number of B cells specific to the pathogen. Some of these cells differentiate into memory B cells, which provide rapid responses to future exposures to the same antigen, while others become plasma cells that secrete antibodies specific to the antigen.