Final answer:
The patient exhibiting immunity to a cold after recovery, and not getting sick again when exposed to the same cold demonstrates natural active immunity, where their immune system developed memory against the pathogen after the initial exposure.
Step-by-step explanation:
The situation where a patient gets a cold, recovers, and then does not get sick again when their classmates catch the same cold is an example of active immunity. Specifically, it is natural active immunity, which is the adaptive immunity that develops after natural exposure to a pathogen. In this case, the patient had a primary immune response to the cold virus which likely involved the production of specific antibodies and memory cells. When re-exposed to the same virus, their immune system rapidly responded, thereby preventing a new infection. This is different from passive immunity, which is a short-term immunity that results when antibodies or activated T cells are transferred from another source, but does not involve the host developing memory against the pathogen.
Passive immunity results when pathogen-specific antibodies or activated T cells are transferred to a person who has never been exposed to the pathogen. Passive immunity provides immediate protection from a pathogen, but the adaptive immune system does not develop immunological memory to protect the host from the same pathogen in the future. Unlike active immunity, passive immunity lasts only as long as the transferred antibodies or T cells survive in the blood. This is usually between a few days and a few months. However, like active immunity, passive immunity can be acquired both naturally and artificially.