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“But I vaccinated those calves for enterotoxemia, they couldn’t have died from Clostridium perfringens. That vaccine must not be any good! Just wait until I get hold of that drug company salesperson.” Does this scenario or a similar experience seem familiar to you? I’m not saying that there aren’t times when vaccines fail to cause the animal to produce its own protective response by stimulating the immune system. A substance foreign to the body that induces the immune system to response is called an antigen, as found in vaccines or organisms such as bacteria or viruses. An animal’s immunological response to a vaccine is acquired. The immune system is composed of humoral and cellular portions both of which respond to an antigen (Table 1).