Plasma cells are a type of white blood cell that produces antibodies. As such, they are an important part of the immune system. They are formed from B cells produced in a person's bone marrow. Once produced, B cells mainly stay within the marrow and wait until an antigen appears in the body. Antigens bind to the cell and stimulate it to form plasma cells. These cells then produces antibodies to destroy the pathogen.
There are five types of white blood cells in the body: nuetrophils, eosinophil, basophil, lymphocytes, and monocytes. Lymphocytes are divided into three types of cells: T cells, natural killer cells, and B cells. T cells migrate to the thymus where they wait until called for. Natural killer cells are those that destroy damaged cells that could grow out of control. All three types of lymphocytes exist to help the body defend itself against disease.
Plasma cells are just one method the body uses to fight diseases. Each produces a particular antibody to neutralize a particular antigen. When an antigen enters the body, it must first bind to a B cell, which then proliferates to form plasma cells. Those cells then secrete antibodies that inactivate the pathogen and mark it for destruction. Normally, a cell will produce antibodies for four to five days and will then die.
When a new antigen enters the body, some time is needed until the body can develop antibodies to fight it. Naive B-cells will first have to be activated in order to be able to respond to a specific disease causing agent. The antigen itself does this while it is flowing through the body. When it encounters a naive B cell, it will bind to it and help trigger clonal selection. Clonal selection is the process by which activated B cells multiply in order to form several clones of itself.
Some of these clones become plasma cells, which are then able to produce antibodies against the disease causing agent. Some clone cells, however, become memory cells and can last in the body for years. Their purpose is to provide a quick response to the antigen should it appear in the body in the future. This way, it will not take the body as long to fight the same disease.