**Question:** Immunity by vaccination is

A. Passive immunity
B. Active immunity
C. Sensitization
D. Allergy

**Correct Answer:** B. Active immunity

**Core Concept:** Vaccination refers to the process of administering a weakened, attenuated, or inactivated form of a pathogen (e.g., bacteria, viruses) or its component parts (e.g., antigens) to an individual to stimulate the immune system to produce a specific immune response. This acquired immunity helps the individual to defend against the actual pathogen when encountered in the future.

**Why the Correct Answer is Right:** Active immunity is the correct term for immunity gained through vaccination because it involves the body's immune system recognizing and responding to the introduced antigens, leading to the production of specific antibodies and immune cells (e.g., T cells, B cells). This acquired immunity provides long-lasting protection against the targeted pathogen.

**Why Each Wrong Option is Incorrect:**

A. Passive immunity (Option A) refers to the immediate protection provided by antibodies or immune cells from another individual (e.g., transfusion, breast milk). Vaccination creates active immunity through the individual's immune system.

B. Sensitization (Option C) involves the immune system becoming aware of or "sensitized" to an antigen, but it does not imply the development of immunity or the production of specific antibodies and immune cells.

D. Allergy (Option D) is an immune response to non-pathogen substances (e.g., food, drugs) that results in hypersensitivity and clinical symptoms, not immunity.

**Clinical Pearl:** Vaccination is an essential preventive measure against various infectious diseases, as it helps train the immune system to recognize, respond, and neutralize the actual pathogen upon encounter, protecting the individual and potentially herd immunity. This process is crucial for controlling the spread of infectious diseases and maintaining public health.