For each of the panels A–D in Figure , identify the most likely component(s) of the immune response indicated by the red arrow, and briefly describe your reasoning. 
Definitions:
Elderly Population
A group within a society consisting of individuals who are considered to be of an advanced age, often categorized as being 65 years or older.
United States
A country in North America consisting of 50 states and a federal district, known for its diverse geography and multicultural population.
Bulgaria
A country located in Southeast Europe known for its rich history, cultural heritage, and geographical diversity, including mountains, plains, seas, and rivers.
Japan
A country located in East Asia, known for its rich culture, technological advancements, and historical significance.
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