Natural immunity describes the state of being able to resist illnesses; such a condition originates in a healthy immune system. This form of immunity is resistance that one inherits from one's parents, in contrast to acquired immunity, which one develops over the course of one's life. Acquired immunity is built up when one suffers from and beats a given illness; memory cells in the immune system essentially learn the illness and remember how to beat it in the future. If an individual was not born with some form of inherited immunity, it is unlikely that he would survive his infancy, as there are many common illnesses that would be deadly to those without any immunity.
One of the major properties of a healthy and effective immune system is the ability to differentiate between self and non-self. Natural immunity confers that ability on an infant to some degree, allowing his immune system to identify and fight threats. If one's immune system were unable to make the distinction between self and non-self, it would likely attack various necessary aspects of the body rather than harmful and antagonistic elements. Natural immunity ensures this is not a problem in healthy infants; their immune systems are already capable of distinguishing self from non-self, of identifying threats, and of fighting a number of common ailments.
The workings of natural and of acquired immunity are closely related to molecules called antigens. Antigens are substances that provoke some form of immune response. Once the immune system confronts a given antigen, it tends to remember it and is able to mount a much more effective defense the next time the antigen that provoked the response appears. Inherited immunity does not require such memory; the immune system is immediately able to effectively respond to certain antigens based, to some degree, on the acquired immune system of an individual's parents.
Immune response can be specific or nonspecific, based on the particular nature of the threat to an individual's health. Specific immune responses are targeted at specific threats, while nonspecific immune responses are effective against a wide variety of issues. Most of the capabilities of natural immunity are focused around nonspecific immune responses that can handle many types of threats. After birth, as an individual is exposed to a variety of illnesses and antigens through sickness or through vaccines, he begins to build up a library of specific, targeted immune responses that allow him to effectively deal with a variety of specific threats.