**Core Concept**
Active immunity is a type of immunity that occurs when the body produces antibodies in response to direct exposure to an antigen, such as through infection or immunization with a vaccine. This process involves the activation of immune cells, including T-lymphocytes and B-lymphocytes, which recognize and respond to specific antigens.

**Why the Correct Answer is Right**
Active immunity is characterized by the production of long-lived memory cells, which allow for a rapid and effective response to subsequent exposures to the same antigen. This is in contrast to passive immunity, which involves the transfer of pre-formed antibodies from one individual to another. The activation of immune cells and the production of antibodies in response to an antigen is a complex process that involves the recognition of antigens by T-cells and B-cells, and the subsequent activation of these cells through a process known as clonal expansion. This process is mediated by various immune cells, including dendritic cells, macrophages, and T-helper cells.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not accurately describe active immunity. While vaccines can induce active immunity, not all vaccines work in this way.
**Option B:** This option is incorrect because it describes passive immunity, not active immunity.
**Option C:** This option is incorrect because it is a characteristic of passive immunity, not active immunity.

**Clinical Pearl / High-Yield Fact**
It's worth noting that active immunity can be induced through various means, including infection, vaccination, and immunization with specific antigens. This type of immunity is highly specific and can provide long-lasting protection against specific diseases.

**Correct Answer: B. Passive immunity is induced when pre-formed antibodies are transferred from one individual to another, whereas active immunity is induced when the body produces antibodies in response to direct exposure to an antigen.**