When an antigen appears, the adaptive immune system response begins with __________.

• A. release of histamine by mast cells
• B. activation of lymphocytes
• C. production of interferons
• D. phagocytosis by neutrophils

Answer: (B)

Verified step by step guidance

1. Understand that the adaptive immune system is a specific defense mechanism that targets particular pathogens after recognizing their antigens.
2. Recall that the adaptive immune response is characterized by the activation of lymphocytes, which include B cells and T cells, upon encountering an antigen.
3. Recognize that other options like release of histamine by mast cells and phagocytosis by neutrophils are part of the innate immune response, which acts earlier and non-specifically.
4. Note that production of interferons is typically part of the innate antiviral response, not the initial step of the adaptive immune system.
5. Conclude that the first step in the adaptive immune response when an antigen appears is the activation of lymphocytes, which then proliferate and differentiate to target the antigen specifically.