Vaccination stimulates ________ immunity.

Choice C rationale

Negative feedback inhibition is the most common mechanism by which target organs regulate pituitary hormone secretion. When the concentration of a hormone secreted by a target gland reaches a sufficient level, it inhibits the release of stimulating hormones from the pituitary and/or hypothalamus, thereby maintaining hormonal homeostasis and preventing overproduction.

Choice A rationale

Positive feedback is less common in endocrine regulation and typically leads to an amplification of the initial stimulus, rather than a return to homeostasis. An example is the surge of luteinizing hormone during ovulation, where estrogen stimulates more LH release.

Choice B rationale

Direct nervous stimulation occurs in some cases, such as the adrenal medulla's release of catecholamines in response to sympathetic preganglionic neurons. However, for target organ regulation of the pituitary, feedback loops involving hormones are the predominant mechanism, not direct nervous stimulation.

Choice D rationale

Reverberation refers to a type of neural circuit where neurons re-excite themselves or upstream neurons, leading to a sustained output. This concept is primarily relevant to neural networks and not a direct mechanism for endocrine feedback from target organs to the pituitary gland.

Choice A rationale

Natural active immunity occurs when an individual's immune system produces its own antibodies and memory cells after direct exposure to a pathogen through natural infection. Vaccination, while stimulating an active response, is not considered "natural" exposure.

Choice B rationale

Artificial active immunity is stimulated by vaccination. Vaccines introduce weakened, inactivated, or components of pathogens into the body, prompting the immune system to produce its own antibodies and memory cells without causing the disease, thereby conferring long-term protection.

Choice C rationale

Natural passive immunity involves the transfer of antibodies from one individual to another, such as from mother to fetus via the placenta or through breast milk. This provides immediate but temporary protection and does not stimulate the recipient's immune system to produce its own antibodies.

Choice D rationale

Artificial passive immunity involves the direct administration of pre-formed antibodies (e.g., antitoxins, immunoglobulins) to an individual. This provides immediate, temporary protection and is used for acute exposures or in immunocompromised individuals; it does not stimulate active immunity.