What is indicated by the arrows?

A. Weeks: While some effector T cells have a short lifespan of only a few weeks during an active infection, memory cells are designed for long-term persistence. A lifespan limited to weeks would fail to provide the sustained protection required for secondary immune responses. Memory cells survive long after the pathogen is cleared.

B. Days: Effector lymphocytes often survive for only a few days after performing their cytotoxic or helper functions. This rapid turnover prevents chronic inflammation once a threat is eliminated. However, memory T cells are specifically differentiated to avoid this programmed cell death and remain viable for much longer.

C. Decades: Memory T cells are remarkably long-lived and can persist in the body for many decades, often for the entire life of the individual. This longevity ensures that the immune system can mount a rapid response if a pathogen is encountered again years later. They maintain the capacity for clonal expansion.

D. Years: Although memory T cells certainly last for years, this choice is less accurate than the broader clinical observation of their multi-decade persistence. Many vaccinations provide protection that lasts 20 to 50 years or more. Their survival is maintained through slow homeostatic proliferation without the need for antigen re-exposure.

E. months: A survival time of only a few months would necessitate frequent re-vaccination to maintain immunity against common childhood diseases. The physiological purpose of the memory T cell is to provide a semi-permanent record of past infections. Their lifespan significantly exceeds the scale of a few months.

A. 20; 32: The primary dentition consists of 20 teeth, including incisors, canines, and molars, which erupt during infancy. The permanent secondary dentition replaces these with 32 teeth, adding premolars and third molars. This represents the standard anatomical formula for human odontogenesis and maturation.

B. 16; 20: These figures underestimate the count for both deciduous and permanent stages of dental development. A child typically possesses more than 16 teeth once the primary set is complete. An adult with only 20 teeth would be considered partially edentulous, missing significant posterior dentition.

C. 28; 20: This choice incorrectly suggests that infants have more teeth than adults. Human dental development involves an increase in total tooth count as the jaw expands to accommodate larger structures. 28 teeth represent a permanent set excluding the wisdom teeth, not the deciduous set.

D. 32; 20: This inversion implies that the deciduous set is larger than the adult permanent set. Deciduous teeth are smaller and fewer in number to fit the pediatric alveolar bone. The adult mandible and maxilla are anatomically designed to support a more extensive 32-tooth array.

E. 32; 32: While some adults have 32 teeth, no infant develops 32 deciduous teeth in a healthy physiological state. The primary dentition lacks the premolars and third molars found in the permanent set. Using the same number for both stages ignores the transition of dental eruption.