Final answer:
Antibodies, or immunoglobulins, are the molecules responsible for humoral immunity, which is part of the adaptive immune response. They circulate in blood and lymph to bind extracellular antigens, neutralize pathogens, and mark them for destruction.
Step-by-step explanation:
The molecules of the adaptive defense system that provide humoral immunity by circulating freely in the blood and lymph, binding to extracellular antigens to inactivate them and mark them for destruction, are known as antibodies (immunoglobulins). These antibodies are secreted by B lymphocytes, or B cells, and can neutralize pathogens, facilitate their phagocytosis, or activate the complement cascade. There are five classes of antibodies, and their production is a crucial part of the adaptive immune response. Specifically, the presence of antibodies in the blood and lymphatic system enables them to encounter and bind to antigens, promoting various defense mechanisms including opsonization, agglutination, neutralization, complement activation, and enhancement of phagocytosis.