Humans and other animals, such as mice, rats, goats or chickens, can make millions of different kinds of antibody protein or 'immunoglobulin'.

The cells that make these antibodies are white blood cells called B-cells which originate in the bone marrow, from where they move to places such as the lymph nodes. Their activation involves other parts of the immune system including some of the CD4 cells that are directly affected by HIV.

Different kinds of antibody are created in B-cells by shuffling specialised DNA sequences, as they mature, to create B-cell lines that each make just one kind of antibody. The shuffling is random, but the immune system selects and promotes the growth of cell-lines that happen to make antibodies, for example, against HIV in someone who is HIV-positive.

In a process called the maturation of antibody response, antibodies can be modified and improved over time. These modifications may include switching from producing antibodies of one class, such as IgM, to another such as IgG. ('Ig' is short for 'immunoglobulin'.) In all, there are five main classes of antibody, with different biological properties; others are IgA, IgD and IgE. (There are further sub-classes, especially within IgG.)

The same principles are now used in the laboratory to create cell-lines that make large quantities of useful antibodies at low cost.

Antibodies can be produced in the laboratory by injecting an animal with an antigen and then extracting antibodies from its blood. This method is used, for example, to create anti-venoms against snakebite.

A more reliable method is to find the cells that produce the antibodies and fuse them with tumour cells, to create long-lived cell-lines that make large amounts of antibody.

Laboratory-produced antibodies based on a single cell-line are called 'monoclonal antibodies,' in contrast to those extracted from blood - like the human immunoglobulins used for the short-term prevention of viral hepatitis.

With genetic engineering, it is now possible to create libraries of artificial antibodies and to select those of interest, without having to use whole animals at all. It is even possible to put the DNA into plants and literally to grow antibodies that can then be harvested for medical or scientific use.