Tissues or cells from another person act as antigens, which is why transplanted tissues are rejected. In some situations, the immune system mistakes its own cells as antigens, which results in autoimmune disease, such as some forms of arthritis and diabetes. Each B lymphocyte produces one specific antibody. For instance, one B lymphocyte will make an antibody to block a virus that causes the common cold, while another produces an antibody that attacks a bacterium that causes pneumonia. When a B lymphocyte encounters its triggering antigen, it gives rise to many large cells known as plasma cells.
Every plasma cell is essentially a factory for producing a unique antibody.
Each of the plasma cells descended from a given B lymphocyte manufactures millions of identical antibody molecules and pours them into the bloodstream. Antibodies belong to a family of large molecules known as immunoglobulins. 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. Immunoglobulin M is very effective in killing bacteria.