Immunity is a process whereby the body recognizes and defends itself against infectious microorganisms (such as viruses, bacteria and fungi) and nonliving substances (like splinters) that appear to be harmful. It involves a complex network of cells and proteins that is activated by exposure to antigens. Once a pathogen is recognized, the immune system sends antibodies that destroy or neutralize the pathogen.
The first step of this process is called innate immunity, which includes barriers like skin and mucous membranes that keep germs from entering the body. Once a pathogen is detected, the innate system rapidly sends antibodies that destroy or neutralize the invading pathogen. This is also known as the primary immune response. The innate system also produces memory B cells and T cells, which will recognize the specific pathogen and mount a secondary response if the pathogen is encountered again.
Active immunity results from a natural infection with the disease organism, or can be artificially acquired through vaccination with a dead or weakened form of the disease organism. The three essential characteristics of active immunity are recognition, specificity and memory. An individual’s immune system is constantly improving its ability to recognize self-molecules that make up the body and to differentiate them from molecules found in infectious microorganisms or toxic substances. These self-recognition processes are guided by a highly coordinated network of signals and cellular interactions, which is determined by a variety of factors including genetics, cell-surface molecules and secreted signaling proteins.
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