Final answer:
Antigens are substances that induce an immune response leading to the production of antibodies by B lymphocytes, which are part of the humoral immune response. The immune system creates memory cells after exposure to an antigen, allowing a more robust response to subsequent exposures, often enhanced with adjuvants. The antibodies produced can neutralize pathogens or mark them for destruction.
Step-by-step explanation:
A substance that, when introduced into an organism, induces an immune response consisting of the production of a circulating antibody is known as an antigen. Antigens may include proteins, carbohydrates, lipids, and nucleic acids. When antigens enter the bloodstream, they can stimulate B lymphocytes, which are a type of white blood cell, to produce antibodies. These antibodies are part of the humoral immune response and are specific to the foreign antigens they bind to.
After initial exposure to an antigen, the immune system creates memory cells which enable a quicker and stronger response upon subsequent exposures. This process includes affinity maturation, where B cells with higher affinity antigen-binding sites are stimulated to proliferate more than their lower-affinity counterparts. The production of antibodies by plasma cells in this manner is referred to as active immunity. Sometimes, chemicals called adjuvants are used to enhance this immune response by provoking a greater antibody production.
Antibodies have various classes, such as IgG, and can perform different actions such as neutralizing pathogens, marking them for phagocytosis, or activating the complement cascade. Disruptions in the immune system, such as in the case of immune deficiencies or hypersensitivities, can result in increased susceptibility to infections or inappropriately directed immune responses.