VACCINES

The immune system can be prepared against microorganisms, so when this immune system comes into  contact again with the same virulent agent it can be quickly recognized and efficiently combated.   

This prophylactic preparation is based in the immune system memory, that allows an adaptive response, both humoral and cellular to act. This is called active immunization or vaccination. There are other ways of inducing immunity in an animal in a given moment, with treatment purposes in mind instead of prophylactic ones. These ways are: passive immunization or serotherapy. This therapy consists on the transference of specific immunoglobulins against an antigen from one animal to another (usually belonging to the same species in order to avoid adverse reactions or rejection). These antibodies are produced by the donor after an active response produced by vaccination or antigenic stimulation. 

Nowadays serotherapy is not used in the porcine species, even though it is still very useful in other species such as dogs (against hard-path disease), cats (panleucopaenia) and horses (tetanus).

A humoral response (exclusively based on antibodies) may be transferred from the donor to the receptor during serotherapy (main biological functions of immunoglobulins). This response is not very durable due to the catabolism of immunoglobulins. 

The discovery of monoclonal antibodies aroused great hopes aroused about the possibilities of using serotherapy, even against some tumors. The fact that hybridomes can only be produced in mice has however, limited the use of these antibodies due to the rejection process.

What is a vaccine?
 

When an infectious agent (dead or alive) is presented to the immune system (either the whole organism or parts of it) and an immune response is produced (humoral or cellular) without primary or secondary lesions, we have a vaccine.    

The first vaccine of which we have scientific data is usually attributed to Edward Jenner (1749-1823) who in 1796 discovered that people that had suffered from cowpox were protected against human smallpox, an often fatal disease. He used vesicles from infected cows to produce a preparation that he later inoculated to healthy people, which were then protected from human smallpox. This is why we call this method vaccination (administration of "vaccinia" virus)  

Any infectious agent (fragmented or as a whole) able to induce an immune response without producing any kind of lesion is considered as a vaccine. 

Louis Pasteur Laboratory-Museum, Paris.

Around 100 years after the first vaccinations, Louis Pasteur (1822-1895) demonstrated that immunity could be induced using homologous microorganisms (Jenner used heterologous microorganisms: cow virus to prevent a human disease). These microorganisms were either modified or inactivated to attenuate their virulence. This was the beginning of inactivated vaccines, used by Pasteur against carbuncle, and of attenuated ones, first produced in 1885 against rabies. Even though Pasteur did not know the mechanisms of immune activation, induction or memory, his work in vaccines has been or great importance up until our days. These vaccines, today known as conventional vaccines, have been very effective in the fight against a large number of animal and human diseases, from smallpox, at the beginning of these studies, to rabies and porcine diseases such as Foot and Mouth Disease and Classical Porcine Fever. 

The immune mechanism of vaccination was finally understood in 1957 thanks to Frank Burnet (1899-1985), who proposed the theory of the clonal selection, and by the later discovery of the role of T and B lymphocytes (1965). 

The immune stimulation by those antigens present in a vaccine induce a primary response with the stimulation or clonal expansion of T and B lymphocytes (memory cells), which are able to induce a secondary response whenever the same antigens enter the organism again. 

How many types of vaccines are available?

The so called new generation vaccines have been produced thanks to the improved knowledge about the immune response and the mechanisms of antigen presentation, the advances in new molecular biology techniques, the identification of important  immunological proteins and the use of different vectors to produce these proteins. Some of these new generation vaccines allow the differentiation between vaccinated and sick animals. However, most of the vaccines now used against a large number of bacterial and viral diseases are still conventional vaccines.  

From a technical point of view, vaccines could be classified in two different groups: 

Throughout this chapter and the following one we will look at each type of vaccine. We will highlight  their advantages and their problems.