Step 1: Understanding the Concept:
Immunity can be classified based on how the body acquires antibodies. Active immunity involves the body producing its own antibodies, while passive immunity involves receiving pre-formed antibodies.
Step 2: Detailed Explanation:
1. Sera (Antisera): These are biological products obtained from the blood of humans or animals that have already been immunized. They contain a high concentration of specific antibodies.
2. Passive Immunity: Because the recipient's body does not produce the antibodies itself, the immunity is "passive." It provides immediate protection but is short-lived (lasting only a few weeks) because the body eventually breaks down the foreign antibodies.
3. Contrast: Vaccines provide active immunity by stimulating the immune system to produce its own defenses and memory cells.
Step 3: Final Answer:
Sera provide immediate, pre-formed antibody defense, which is defined as passive immunity.