Where are specific lymphocytes activated after an antigen is injected into peripheral tissue? Select the most appropriate anatomical site for primary activation.

Introduction:
Antigen location determines where adaptive immune responses initiate. After antigens enter peripheral tissues via injection or injury, antigen-presenting cells transport processed antigen to regional lymph nodes where naive lymphocytes are primed.

Given Data / Assumptions:

• Antigen is introduced into peripheral tissue (not directly into blood or mucosa).
• Dendritic cells migrate via afferent lymphatics to nearby nodes.
• Naive T and B cells recirculate through secondary lymphoid organs.

Concept / Approach:

The immune system compartmentalizes surveillance. Tissue antigens drain to regional lymph nodes; blood-borne antigens are filtered by the spleen; mucosal antigens are sampled by MALT. Matching the route of entry to the corresponding organ identifies the activation site.

Step-by-Step Solution:

Verification / Alternative check:

Clinical observation: ipsilateral lymphadenopathy near injection sites reflects activation and clonal expansion in local nodes.

Why Other Options Are Wrong:

Option A: Blood is a conduit, not the primary activation site.

Option C: MALT specializes in mucosal antigen (e.g., gut, respiratory tract), not typical intradermal/subcutaneous injections.

Option D: The spleen focuses on antigens in the bloodstream.

Option E: The thymus is for T cell development, not antigen-driven activation.

Common Pitfalls:

Assuming all antigens activate in the spleen; forgetting route-specific immune organ specialization.

Final Answer:

Draining lymph nodes