B lymphocytes, commonly known as B cells, are crucial components of the adaptive immune system, specifically involved in humoral immunity. This form of immunity targets and eliminates extracellular pathogens, which are harmful microorganisms located outside host cells. B cells achieve this by producing antibodies, specialized proteins that identify and neutralize these pathogens.

To understand the activation of B cells, it's important to recognize the role of antigen-presenting cells (APCs), such as dendritic cells. These cells express Major Histocompatibility Complex (MHC) class I and class II molecules, enabling them to activate both cytotoxic and helper T cells. When helper T cells are activated, they can subsequently stimulate B cells, initiating their immune response.

B cells develop in the bone marrow, one of the primary lymphoid organs, before migrating to secondary lymphoid organs as naive or inactive B cells. These naive B cells have yet to encounter an antigen. Upon encountering their specific antigen, they internalize and process it, displaying fragments on their MHC class II molecules. This presentation allows effector helper T cells to recognize the antigen and release cytokines, which activate the B cells.

Once activated, B cells undergo proliferation, creating clones, and differentiate into two main types: memory B cells and plasma cells. Memory B cells are essential for a rapid response during subsequent infections, while plasma cells are the effector B cells responsible for producing and secreting antibodies. These antibodies perform various immune functions, including neutralizing pathogens and marking them for destruction by other immune cells.

As the course progresses, further details regarding the activation of B cells, the differentiation into memory and plasma cells, and the specific roles of antibodies will be explored, enhancing the understanding of the adaptive immune response.

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