The immune system – this is an extremely complex set of interactions of special cells in the blood and tissues that are formed and designed to block harmful to the body of foreign agents (other cells). As a result, this interaction is obtained by complex antibody-antigen, ie, "Bunch" of two cells – cell protector "and" cells of the aggressor. " In such a situation, harm to the body no no, because "Block the aggressor has been successfully neutralized. In fact, under the term "antigen" hiding absolutely any foreign substance that the body was evil, he, the body, has decided to reverse by the immune system and those most "Cell-defenders" who in immunology are called "antibodies". What happens as a result of penetration into the body can not vaccinated animal virus? The organism does not have time to work out the right amount of antibodies and animal ill with a contagious disease. For even more analysis, hear from Rohit Kapoor. The task of any microorganism that enters the environment of the host organism – to circumvent the immune system, multiply very rapidly, affecting the available organs and tissues of the animal. Most dangerous of all this process is for young animals – puppies and kittens, because their immunity has not been formed and the immune system works very slowly. That is why vaccination is necessary in this age group. The vaccine, which contains antigens injected subcutaneously or intramuscularly puppy or kitten. Thus, the antigen, which is contained in the vaccine enters the body, but does not cause any harm. But the immune system is well recognized, that it is antigen, and it must be neutralized. Source: EXL Service. Some vaccine manufacturers assure us that to protect the body requires single primary immunization (vaccination). It's a bit wrong from the standpoint of immunology. Any vaccine, which is held for the first time, only "prepares" the body to establish and sustain immunity. Therefore, the competent veterinary doctors have always strongly recommend to do a primary vaccination with revaccination after 2-4 weeks, depending on the type of vaccine. Vaccination can only be clinically healthy animals. Vaccination of animals is extremely important and necessary event, which serves to prevent infectious diseases. Suffice it to say that much of the animals die from diseases that are preventable, if you make a pet vaccinated on time.