The word phagocyte means 'cell eater' and this is exactly what it does (see left) in a process called phagocytosis.
In this process the phagocyte is chemically attracted to a particle. It then travels to the site of this and will engulf it. Once inside the phagocyte, the particle is digested by enzymes and exits the cell.
An antigen is something that causes lymphocytes to produce antibodies. Antibodies are specific for a particular antigen. They may stick to the surface of an antigen, making them clump together or burst. They could also make it more vulnerable to phagocytes.
B lymphocytes carry antibodies exposed on their cell surface. There are at least 100 million genetic types, programmed to recognise any foreign molecule or antigen that exists in nature.
When it recognises an antigen that matches it's antibody; rapid division takes place that produces antibody secreting cells. This is called the primary immune response.
In division, some smaller but longer living memory B lymphocytes are produced. These will float around the body ready to reproduce rapidly should the antigen come again: this is the secondary immune response. These cells provide immunity.
The graph shows how the production of antibodies is much more rapid in the secondary response because of memory cells.
If a vaccination is given several times to boost its effectiveness, point X shows the best point for the second vaccination to take place, since it encourages a secondary response during a slump in antibody production.
Passive immunity is the injection of ready-made antibodies to prevent disease. These were conventionally produced by animals, but this is expensive, cruel and complicated.
Monoclonal antibodies are produced in a laboratory by fusing a lymphocyte with a tumor cell.
A tumor cell is used because of its ability to divide continuously so the fused cell (called hybridoma) can be produced on a large scale.
These monoclonal antibodies are being used increasingly in medical treatment because they can target specific molecules related to the disease process.