What process do B cells undergo when presented with an antigen on a class II major histocompatibility complex (MHC) protein?

When B cells encounter an antigen presented on a class II major histocompatibility complex (MHC) protein, they undergo a process known as blast transformation. During this process, B cells are activated and differentiate into larger cells known as blasts. This transformation is crucial for the immune response as it marks the activation phase where the B cells prepare to produce antibodies specific to the encountered antigen.

Upon binding to the antigen presented by the MHC class II molecule, the B cell internalizes the antigen, processes it, and then presents its fragments on its own MHC class II molecules. This interaction is critical for the full activation of the B cell, often aided by T helper cells, which can provide additional signals for the B cell’s proliferation and antibody production.

Ultimately, blast transformation is a pivotal step in the immune response, enabling B cells not only to proliferate but also to optimize their ability to recognize and combat pathogens, solidifying their role in adaptive immunity. Through this transformation, they significantly enhance the immune system's ability to remember and respond to specific antigens in future encounters.