Notes on Cells of the Innate Immune System

Overview of Cells of the Innate Immune System

Introduction

  • Leukocytes (White Blood Cells): Serve as defenders against infections by patrolling the body.

  • Mobility: Move through blood and lymphatic systems, can exit circulation to tissues.

Classification of Leukocytes

  1. Morphology: Classified by the number of lobes in nuclei and presence of granules.

    • Granulocytes: Multi-lobed nuclei and prominent cytoplasmic granules.

    • Agranulocytes: Single, unlobed nuclei with few or no granules.

Functions of Leukocytes

  • Direct Combat: Some kill invaders directly.

  • Produce Soluble Molecules: Others produce molecules that deter infection.

  • Autonomous: Some act independently, while others require signals from other cells.

  • Regulation: Certain leukocytes coordinate immune responses.

Origin of Leukocytes

  • Hematopoietic Stem Cells: All leukocytes derive from these pluripotent cells in bone marrow.

  • Lineages:

    • Myeloid Lineage: Includes granulocytes and monocytes/macrophages.

    • Lymphoid Lineage: Includes B cells, T cells, NK cells.

Types of Leukocytes

Granulocytes

  1. Neutrophils: 60% of leukocytes; effective against bacteria; multi-lobes.

  2. Basophils: Participate in allergic reactions; bilobed nuclei, release histamine.

  3. Eosinophils: Respond to parasitic infections; bilobed, granules stain with eosin.

Agranulocytes

  1. Lymphocytes: Includes T cells, B cells, and NK cells. Critical for adaptive immunity.

    • B cells: Produce antibodies; different types include plasma cells.

    • T cells: Differentiate in the thymus; activated by antigens presented by MHC.

    • NK cells: Attack abnormal host cells without prior sensitization.

  2. Monocytes: Become macrophages when in tissues; responsible for phagocytosis.

  3. Dendritic Cells: Act as antigen-presenting cells, crucial for T cell activation.

Immune Defense Mechanisms

Phagocytosis

  • Definition: Engulfment of pathogens by phagocytes (e.g., macrophages, neutrophils).

  • Steps: Attachment -> Ingestion -> Digestion of the pathogen.

Inflammation

  • Cardinal Signs: Heat, redness, swelling, and pain.

  • Functions: Traps pathogens, promotes healing, and prepares for adaptive responses.

Complement System

  • Function: A key component of the immune response; aids in pathogen lysis and opsonization.

  • Activation: Occurs via the classical, alternative, or mannan-binding lectin pathway.

Leukocyte Interactions and Signaling

Cytokines and Chemokines

  • Cytokines: Protein signals that mediate and regulate immunity, inflammation, and hematopoiesis.

  • Chemokines: Type of cytokine that induces chemotaxis allowing leukocyte movement.

Cell Adhesion Molecules

  • Integrins: Facilitate stable attachment between leukocytes and endothelium.

  • Selectins: Help in leukocyte rolling and migration.

T Cell Activation

  1. Clonal Expansion: T cells proliferate upon activation by recognizing antigens.

  2. Memory Formation: Some T cells become memory cells for faster responses in future exposures.

Conclusion

Understanding the diverse roles of leukocytes in both innate and adaptive immunity is essential for grasping how the body defends against infections. Each leukocyte type has specific functions that contribute to a robust and coordinated immune response. Their interactions within tissues and organs highlight the complexity of the immune system.