Study Notes for BM212 Immunology Lecture 2


Learning Objectives

  • Understand the basis of immune anatomy

  • Identify key cells involved in innate and adaptive immunity

  • Comprehend the importance of lymphocyte recirculation in the immune system

  • Describe the basic structure of lymphoid organs

Cells of the Immune System

  • Neutrophils:

    • Function: Phagocytize and kill bacteria; mediate inflammation.

  • Eosinophils:

    • Functions: Release chemicals targeting parasites; phagocytize certain parasites; participate in allergic responses.

  • Monocytes:

    • Function: Develop into macrophages.

  • Macrophages:

    • Functions: Phagocytize microbes; mediate inflammation; present antigens to T cells.

  • Basophils:

    • Function: Enter tissues at the site of injury; secrete anticoagulant heparin.

  • Natural Killer Cells:

    • Function: Attack cancerous and virus-infected cells; also involved in specific immunity.

  • Dendritic Cells:

    • Similar function to macrophages; essential in antigen presentation.

  • Mast Cells:

    • Function: Secrete histamine during inflammatory responses; participate in allergic reactions.

  • Note: Only certain cells are crucial for innate immunity.

Key Cells in Adaptive Immunity

  • Lymphocytes (20-40% of WBC):

    • B Cells: ~23%

    • Function: Produce antibodies.

    • T Cells:

    • CD8+ Cytolytic T Cells: ~20%

      • Function: Kill virus-infected cells.

    • CD4+ Helper T Cells: ~45%

      • Function: Help other immune cells.

    • Gamma Delta T Cells: ~5%

      • Role in immune response varies.

T Cell Development and Function

  • T cells are produced in the bone marrow.

  • Selected within the thymus.

  • Exit via lymphatics, which drain peripheral tissues towards lymph nodes.

  • Lymph Nodes:

    • Aggregates of T cells, B cells, and antigen-presenting cells (APCs).

    • Function: Act as headquarters to determine when and where immune responses occur.

    • Concentrated in areas like the nose, mouth, lungs, and gut.

Structure of Lymph Nodes

  • Key entry and exit points for immune cells:

    • Blood supply provided via arteries and veins, including entry of lymphocytes through HEVs (High Endothelial Venules).

    • Main entry is through afferent lymphatic vessels.

  • Key Areas within Lymph Nodes:

    • Germinal Center: Houses B cells.

    • Paracortical Area: Primarily T cells.

    • Parafollicular Area: Interface for T and B cell interaction.

    • Medullary Cords: House plasma cells producing antibodies and macrophages.

Interaction within Lymph Nodes

  • Dendritic Cells (DCs):

    • Circulate through blood and lymphatics; bring antigens to lymph nodes via afferent lymphatics.

  • Interaction occurs in the follicular region where B cells can present antigen to T cells and vice versa.

  • This interaction activates T cells that assist B cells.

  • Activated T and B cells exit via efferent lymphatics to address infection elsewhere in the body.

Other Lymphatic Organs

  • Spleen:

    • Red Pulp: Site where RBCs are broken down.

    • White Pulp: Region for interaction between B, T cells, and APCs.

  • Peyer's Patch:

    • Organized structures in the gut where dendritic cells can sample antigens and interact with T cells, which may activate B cells for antibody production.

    • M-cells serve as specialized APCs for presenting antigens.

Lymphocyte Exit from Nodes

  • Dendritic cells enter via afferent lymphatics.

  • T cells enter via HEVs; those recognizing antigen will leave via efferent lymphatics, while non-responsive cells exit via cortical sinuses.

Lymphocyte Recirculation

  • Lymphocytes must leave peripheral lymphoid organs to reach sites of action.

  • The process involves:

    1. Dendritic cells carrying antigens travel to lymph nodes.

    2. Interact with naïve T cells.

    3. Activated T cells return to the bloodstream after activation.

Recognition in the Immune System

  • Innate Immune System:

    • Characteristics: Inherited, non-specific responses initiated without prior exposure to invaders.

    • Functions: Include phagocyte recognition of common microbial motifs; generate responses quickly (within minutes).

    • Initiation: Begins with recognition of pathogens via PAMPs (Pathogen-Associated Molecular Patterns), triggering inflammation, recruiting cells, and activating adaptive immunity.

  • Adaptive Immune System:

    • Characteristics: Specific immunity tailored to each pathogen, requiring prior exposure and taking longer to establish but creates immunological memory.

    • Components: Involves specific cellular responses, memory formation, and differentiation of immune cells.

Pathogen Recognition and Receptor Types

  • PAMPs:

    • Structural motifs common in many pathogens but not found on host cells.

  • DAMPs:

    • Motifs from damaged host cells that indicate injury.

  • PRRs (Pattern-Recognition Receptors):

    • Receptors, like Toll-like receptors (TLRs), facilitate detection of PAMPs and DAMPs.

  • Types of PRRs:

    • TLRs: Recognize diverse pathogens and initiate immune responses.

    • C-type lectin receptors (CLRs): Recognize carbohydrates.

    • NOD-like receptors (NLRs): Recognize intracellular bacteria.

Cytokine Function and Types

  • Cytokines:

    • Definition: Hormone-like proteins important for immune communication, involved in a variety of immune responses including infection and inflammation.

    • Types: Include chemokines, interferons, interleukins, and tumor necrosis factors.

    • Functions: Modulate target cell functions including migration, activation, expression of surface molecules, and apoptosis (cell death).

  • Chemokines:

    • Function: Subtype of cytokines responsible for attracting immune cells to infection sites; examples include IL-8 and RANTES.

Key Definitions and Concepts

  • Innate Immunity Definition:

    • Provides immediate defense against infections, operates without prior exposure to pathogens.

  • Adaptive Immunity Definition:

    • Provides tailored responses against specific pathogens after learning from past encounters, leading to long-term immunity.

Learning Outcomes

  • Understanding the necessity of lymphocyte recirculation in the immune response.

  • Describing the structural characteristics of lymphoid organs.

  • Recognizing major anatomical components of the innate and adaptive immune systems.