Antibodies ambush antigens circulating in the bloodstream. However, they are powerless to penetrate cells. Each B cell is programmed to make one specific antibody. For example, one B cells will make an antibody that blocks a virus that causes the common cold, while another produces an antibody that attacks a bacterium that causes pneumonia. When a B cell encounters its triggering antigen, it gives rise to many large cells known as plasma cells.
Every plasma cell is essentially a factory for producing antibody.
Each of the plasma cells descended from a given B cell manufactures millions of identical antibody molecules and pours them into the bloodstream. An antibody matches an antigen much as a key matches a lock. But whenever antibody and antigen interlock, the antibody marks the antigen for destruction. Different types play different roles in the immune defense strategy. Immunoglobulin G, or IgG, works efficiently to coat microbes, speeding their uptake by other cells in the immune system.