Final answer:
B cells require two signals for activation: first is the recognition of the antigen by B cell's surface immunoglobulin, and second is the engagement with the T cell's cytokines.
Step-by-step explanation:
Lymphocyte activation, specifically for T and B cells, requires two types of signals for an effective immune response. For B cell activation by a T cell-dependent antigen, the B cell's surface immunoglobulin must recognize the native antigen, which is then internalized, processed and presented on a class II MHC molecule to Th2 cells.
The T cell, recognizing the antigen, binds with its antigen receptor and gets activated, secreting cytokines that diffuse to the B cell to complete its activation.
The B cell therefore receives the first signal through its surface antibody binding to the antigen and the second signal from the T cell via cytokines. This complex interaction between B and T cells is crucial for the adaptive immune response.
In the case of T cell-independent antigens, B cells can become activated without T cells, usually through direct recognition of repeating epitopes on antigens and a second signal, potentially from interactions of Toll-like receptors (TLRs) with pathogen-associated molecular patterns (PAMPs).