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The immune system can be thought of as a surveillance system to discriminate between “self" and "non-self." From simple life forms such as insects to advanced life forms such as humans, all living organisms have some form of host defense mechanism. Most have multiple, overlapping mechanisms ranging from very non-specific resistance to highly specific immunity. White blood cells forming "pus" around a splinter is representative of non-specific host resistance to invasion while immunity to IBR virus after vaccination or following recovery from active infection is representative of specific active immunity.
These host defense systems protect livestock and man from the millions of microbes that attack the body every day. Without them, living creatures would die rapidly from the constant, and sometimes successful, attempts by disease agents to invade the body.
Host defense mechanisms also protect livestock and man from invaders within. For example, abnormal cells such as cancer cells are detected and destroyed every day by this surveillance system. The abnormal cells may also be those infected with viruses. Viruses cannot multiply by themselves and require living cells to do their replicating. The immune system detects and destroys these virus-altered cells.
Most producers have some knowledge of the immune system. They know that with certain vaccines, livestock can be protected from infectious disease. They also are aware that the immune system has "memory” - the ability to remember past disease or past vaccinations for a long period of time. For example, calves given blackleg vaccine at branding and weaning are generally immune to this disease for life.
Producers are also aware that the immune system is under genetic control. They have seen that cross-bred calves and yearlings are more disease resistant than most purebreds. From news reports, they know that you cannot transplant a kidney or heart from one person to another unless the donor and recipient are genetically very similar in blood and tissue type, factors controlled by our genes. Even then, the body still recognizes the donor organ as slightly foreign, so the immune system must be suppressed by drugs to prevent rejection. So, producers are aware that the immune system can differentiate “self" from "non-self."