Humoral adaptive immunity is an integral part of the adaptive immune system, which mounts a highly specific defense against pathogens but takes a longer time to respond (compared to the innate immune system). Humoral immunity is the arm of the immune system protecting the extracellular fluids of the lymphatics (lymph), interstitium, and circulatory system (plasma) from microbial contamination mediated through soluble molecules. The B cells play a major role, producing antibodies or immunoglobulins. Arising from the bone marrow, B cells originate from the common lymphoid progenitor and undergo stages to assemble the B cell receptor. To become fully functional, activation follows, and this can be T cell–dependent (which produces memory cells) or T cell–independent (producing a short-lived response). When activated, B cells go through processes enhancing antigen affinity, class switching, and differentiation to plasma cells and memory cells. Plasma cells produce the antibodies, while memory B cells respond to reinfection. There are different immunoglobulin isotypes, generally providing immune protection through complement activation, opsonization, neutralization of toxins or viruses, and induction of cell lysis.