Article Sections

Introduction

The immune system is complex, with overlapping pathways.  This article provides an overview of the immune system by conceptualizing it based on 2 major branches:

  • Innate immunity refers to a rapid generalized response that remains constant with each exposure.  It occurs at the site of infection.
  • Adaptive immunity refers to a slower, memory-based response to specific pathogens that improves over time.  It begins when antigen-presenting cells (APCs [eg, dendritic cells]) move from the site of infection to the nearest lymphoid tissue (eg, lymph node).  The adaptive immune branch is subdivided into 2 categories:
    • Cell-mediated immunity (T cells) is characterized by direct killing of intracellularly infected or abnormal cells (cytotoxic T-cell response) and broad assistance of other immune cells (helper T-cell response).

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