Define antibodies and antigens, and describe the different ways in which the body develops immunity.

Immunity is the capacity of the body to recognise the intrusion of disease-causing microorganisms (pathogens) and develop a response to fight infection. Antigens and antibodies are crucial to the process of acquiring immunity. Each cell has antigens in the cell surface membrane, which act as markers to other cells. The body identifies foreign cells by their antigens and makes antibodies in response. Antibodies are proteins produced by lymphocytes, which with specific antigens, neutralise or destroy them. Antibodies are acquired through active or passive pathways.Active immunity is when the body produces it's own antibodies to help fight infection. Natural active immunity is acquired through exposure to antigens when the body is naturally exposed to a disease. The body produces T memory cells, which make it immune to the disease in the future. Natural artificial immunity is the injection of weakened or dead antigen fragments, thus the body is exposed to disease and produces T memory cells to fight off future infections. Passive immunity is the transfer of antibodies into the body. Natural passive immunity is acquired when antibodies cross from the mother to the fetus across the placenta or to the baby through breast milk. Artificial passive immunity describes when antibodies are injected into the body to provide immediate protection against a pathogen.