Final answer:
Naive B cells initially express B-cell receptors (BCRs) that are membrane-bound forms of IgM and, to a lesser extent, IgD. These receptors are crucial for the early recognition of antigens and the initiation of immune responses. Upon encountering an antigen, naive B cells can differentiate into plasma cells, which secrete antibodies, participating in the adaptive immune response.
Step-by-step explanation:
Antigen Receptors of Naive B cells
The antigen receptors (Ag receptors) of naïve B cells are composed of membrane-bound monomeric forms of IgD and IgM, which are specialized to bind antigen epitopes specifically through their Fab antigen-binding regions. These B-cell receptors (BCRs) are crucial for recognizing pathogens or their components and initiating a cascade of immune responses. When a naïve B cell encounters its specific antigen, it can differentiate into a plasma cell that secretes antibodies, or into a memory cell that provides long-term immunity.
Naïve B cells generally express IgM as their primary B-cell receptor, with a smaller subset also expressing IgD. The IgM molecules are vital for the early stages of immune response and are capable of forming pentamers that can bind ten identical antigens, providing a strong initial immune reaction. IgM and IgD BCRs facilitate the engagement and processing of antigens, and upon activation, naive B cells undergo clonal expansion and differentiation, thanks to the interaction with helper T cells and other signals.
Fulfilling their role in the adaptive immune system, these cells are known as antigen-presenting cells (APCs) and contribute significantly to the humoral immune response by producing specific antibodies against antigens. The significance of IgM in the immune response can be attributed to its pentameric structure that enables high avidity antigen binding, though not as stable as the binding seen with IgG antibodies.