Hematopoietic stem cells reside in the bone marrow, and constantly receive signals to produce new hematopoietic cells (Immune cells). If B cells are needed, stem cells will become pre-B cells which will quickly try to generate a B cell receptor.
The B cell receptor, or BCR, is the specialized receptor that confers B cells their specificity. Each B cell can recognize one ‘shape’ (say, the shape of a flu protein) because their BCR is specific for that ‘shape’. The specificity of each BCR is generated randomly, and the large number of B cells in the body ensures that regardless of what is presented to the immune system, there will always be a few B cells which can recognize it. B cell receptor generation occurs through a process called VDJ recombination.
A B cell that successfully produces a B cell receptor, and leaves the bone marrow is termed a naive B cell. Once out of the bone marrow, naive B cells will travel to secondary lymphoid organs such as the spleen or lymph nodes, and they will search for the specific antigen which their B cell receptor is specific for. If they find the antigen (Imagine a flu-specific B cell finds flu), the B cell will begin to rapidly divide and create new copies of itself, a process called clonal expansion.
As naive B cells divide in response to their antigen, they will differentiate into either effector B cells (also termed plasma cells), or memory B cells. Plasma cells are responsible for producing large amounts of antibody, all specific for the antigen that the naive B cell was specific for. These antibodies can either kill their target directly, or guide other immune cells such as macrophages or natural killer cells to more effectively recognize and kill the foreign antigen. Plasma cells are important for the clearance of pathogens through their antibody production, but a relatively short lived, and so will die after the infection has been resolved – termed contraction.
Memory B cells, while still capable of producing antibody, produce less than plasma cells. They are, however, much longer lived, and can potentially persist in the body for decades. If the same host is infected with the same pathogen, specific memory B cells will ‘remember’ the pathogen, and respond by releasing antibody and rapidly creating new plasma cells. This memory response leads to quick clearance of the pathogen, often even before the host will realize the infection is present.