Final answer:
An antibody is a protein produced by plasma cells that helps in the immune response by binding to antigens on pathogens and marking them for destruction. B cells are responsible for producing antibodies, which can neutralize pathogens and prevent infection.
Step-by-step explanation:
An antibody, also known as an immunoglobulin (Ig), is a protein that is produced by plasma cells after stimulation by an antigen. Antibodies are the functional basis of humoral immunity. Antibodies occur in the blood, in gastric and mucus secretions, and in breast milk. Antibodies in these bodily fluids can bind pathogens and mark them for destruction by phagocytes before they can infect cells.
B cells are immune cells that function primarily by producing antibodies. Once activated by binding to an antigen, B cells differentiate into plasma cells, which produce and secrete antibodies specific to that antigen. Antibodies help in the immune response by binding to antigens on pathogens, marking them for destruction by other immune cells such as phagocytes. They can also neutralize pathogens directly, preventing them from infecting cells.
The production of antibodies by plasma cells in response to an antigen is called active immunity. On the other hand, passive immunity occurs when antibodies are introduced into the body from an external source, providing immediate protection without the need for the individual's own immune response.