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Global Immunization

In addition to developing new safe and effective vaccines, a major challenge for the future is delivering available vaccines to worldwide populations. When this was done in the case of smallpox, the disease was eliminated from the world; and poliomyelitis is almost eradicated now. The World Health Organization and the governments that support it deserve much of the credit for these achievements. Much remains to be done, however, and a recent gift of a billion dollars from Bill Gates, one of the founders of Microsoft, and his wife will go a long way to implementing further progress in this area.

To immunize universally, it is necessary to have vaccines that are easily administered, inexpensive, stable under a variety of environmental conditions, and preferably painless. Instead of expensive needles and syringes and painful injections, vaccines may be delivered in a number of easy ways. For example, naked DNA is coated onto microscopic gold pellets, which are shot from a gunlike apparatus directly through the skin into the muscle, painlessly. Skin patches deliver antigens slowly through the skin. Vaccines against mucous membrane pathogens can be delivered by a nasal spray, as in some new influenza vaccines, or by mouth. Time-release pills introduce antigen steadily to give a sustained immune response.

In addition to sprays and pills that deliver antigens to mucous membranes in order to induce mucosal immunity, techniques are being developed to get antigens directly to M cells in the gastrointestinal mucosa. These are the cells that can endocy-tose antigens and deliver them across the membrane to the lym-phoid tissue of the Peyer's patches, where immune responses occur. Antigens are incorporated into substances known to bind to M cells, thereby facilitating entry into the M cells and the Peyer's patches. ■ M cells, p. 397

Another promising means of immunization, especially for developing nations, is the use of edible vaccines produced in plants. Preliminary studies have shown that genes from various pathogenic organisms can be introduced into plants. For example, potato plants were genetically engineered with a gene for part of an Escherichia coli exotoxin that affects the gastrointestinal tract. The potatoes expressing the harmless fragment of the exo-toxin were then fed to mice, inducing the production of serum and secretory antibodies against the toxin. It is hoped that appropriate genes can be introduced into common plants, such as tomatoes and bananas, resulting in very low cost immunization for whole populations. These investigations, however, are in their early stages.

One of the major remaining challenges is development of an effective vaccine against HIV that can be administered universally. Although many anti-HlV vaccines are under investigation and in clinical trials, there is no indication that a truly effective immunization program for HIV disease is imminent. It is also probable that when HIV disease is brought under control, a new and currently unforeseen challenge will arise during the twenty-first century.

438 Chapter 17 Applications of Immune Responses

17.1 Principles of Immunization (Figure 17.1) Active Immunity

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