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How do B lymphocytes respond when they encounter a foreign antigen?

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When a B lymphocyte encounters a foreign antigen, it undergoes a process of activation and clonal expansion to generate cells with identical antigen specificity.1

In most cases, B-cell activation is T-dependent, requiring stimulation from an activated helper T cell. This involves a cell-to-cell interaction between the CD40 protein on the B cell and the CD40 ligand on the helper T cell, supplemented by cytokines secreted by the T cell. Some antigens, typically large polymers found on bacteria, are T-independent and can stimulate B cells directly.23

Once activated, B cells differentiate into two primary types:
* Plasma cells: These cells secrete large quantities of antibodies into the blood and lymph to neutralize or destroy the target antigen.13
* Memory B cells: These cells provide long-lasting immunity. They do not secrete antibodies initially but persist in the body. If the same antigen is encountered again, they trigger a faster and more robust immune response, often preventing illness.13