What type of immunity is characterized by a rapid decrease over time?

Passive immunity is characterized by a rapid decrease over time because it involves the transfer of antibodies from one individual to another, rather than the recipient's immune system generating its own antibodies. This form of immunity can be acquired naturally, such as when antibodies are passed from mother to infant through breast milk, or artificially, such as through the administration of antibody-rich blood products or immunoglobulins. Since these antibodies are not produced by the recipient's own immune system, they provide immediate protection against infections but do not confer long-term immunity, leading to a decline in their effectiveness once they are metabolized or cleared from the body.

In contrast, active immunity involves the body's own immune response to exposure to a pathogen or through vaccination, which creates memory cells that can provide long-lasting protection. Cellular immunity and humoral immunity are types of active immunity and are generally more durable as they rely on the body’s production of specific immune cells or antibodies that persist over time. Therefore, while passive immunity provides immediate but transient protection, active forms of immunity tend to last much longer, demonstrating the main distinction in how they function and the duration of their effectiveness.