Final answer:
Acquired immunity that develops after exposure to an antigen is known as active immunity. This can be natural, as with recovery from a disease, or artificial, like the immunity gained from vaccination.
Step-by-step explanation:
The form of acquired immunity that develops in response to an immunogen (antigen) is known as c) Active immunity. This occurs when the adaptive immune system responds to a pathogen by developing an immunological memory. This active immunity can be either natural, as seen when a person recovers from an infection like a cold and develops immunity against it, or artificial, which is triggered by vaccination. Vaccination leads to artificially acquired active immunity, where the vaccine, usually consisting of a killed or weakened pathogen, stimulates the immune system to develop memory cells without causing a full-blown disease. Another example of active immunity is when someone has an illness and then recovers, as in the case of the patient who got a cold and then did not become re-infected when classmates fell ill. This is natural active immunity, gained through exposure to the actual pathogen.
On the other hand, an example of natural immunity that is passively acquired would be the antibodies a baby receives from its mother either through the placenta during pregnancy or through breast milk after birth. This type of immunity is temporary and does not involve the individual's immune system creating a memory of the pathogen.