Host Defenses and Immunology

Lymphatic System and Host Defenses

  • Lymphatic vessels: channels through which lymph is transported back to venous circulation.
  • Lymph nodes: clusters of lymphocytes that play a critical role in immune response.

Immunology Basics

  • Immunology: Study of host defenses against foreign substances. It deals with how organisms respond to antigens and distinguish self from non-self.
  • Foreign materials include:
    • Bacteria
    • Viruses
    • Viroids
    • Satellite viruses
    • Prions
    • Malignant cells
    • Transplanted tissues

Types of Immunity

  1. Innate (Natural) Immunity:

    • Nonspecific defenses against any foreign material.
    • Activates immediately upon exposure.
    • The response is consistent with each encounter of the same invader.

  2. Acquired (Adaptive) Immunity:

    • Specific defenses developed against particular foreign materials.
    • Involves memory, leading to a quicker and stronger response on subsequent encounters with the same antigen.

Layers of Host Defenses

  • A multilevel network combining innate and adaptive immune responses.
Innate Host Defenses
  • First line of defense:
    • Surface protection through anatomical and physiological barriers (skin, mucous membranes).
  • Second line of defense:
    • Cellular and chemical responses activated if pathogens breach surface barriers (inflammation, phagocytosis).
  • Third line of defense:
    • Specific defenses requiring development for each microbe (T and B cells).

Functions of the Immune System

  • Surveillance to detect and respond to foreign materials.
  • Recognition of self vs. non-self is crucial for avoiding autoimmune responses. Autoimmunity can cause diseases like rheumatoid arthritis.

Recognition Mechanisms

  • Major Histocompatibility Complex (MHC):
    • Markers on self cells enabling immune system recognition.
  • Pathogen-Associated Molecular Patterns (PAMPs):
    • Found on non-self cells, recognized by Pattern Recognition Receptors (PRRs) on host cells.

Immune Cells Overview

  • Hematopoietic Stem Cells:
    • Origin of all blood cells in the bone marrow. Differentiate into various leukocytes:
    • Agranulocytes:
    • Monocytes, Lymphocytes
    • Granulocytes:
    • Eosinophils, Basophils, Neutrophils
Key Types of Leukocytes
  • B cells: Produce antibodies and differentiate into plasma cells.
  • T cells:
    • Cytotoxic T cells kill infected cells.
    • Helper T cells support immune responses.
  • Macrophages: Phagocytic cells that engulf and destroy pathogens.

Organs and Tissues of the Immune System

  1. Primary Lymphatic Organs:
    • Thymus and Bone Marrow; sites for lymphocyte maturation.
  2. Secondary Lymphatic Organs:
    • Spleen (filters blood) and Lymph nodes (filter lymphatic fluid).

First Line of Defense: Physical Barriers

  • Skin: Thick, waterproof barrier; contains microbiota that outcompete pathogens.
  • Mucous Membranes: Trap invaders with enzymes like lysozyme and lactoferrin, produce mucus to flush out pathogens.

Second Line of Defense: Chemical Mediators

  • Lysozymes: Enzymes that lyse bacterial cells.
  • Cytokines: Signaling proteins that mediate immune responses.
  • Histamines: Released by basophils and mast cells to promote blood vessel dilation (inflammation).

Phagocytosis Steps

  1. Chemotaxis and Adhesion: Phagocytes are recruited to infection sites.
  2. Engulfment: Phagocytes engulf pathogens within phagosomes.
  3. Destruction: Phagolysosomes form and digest pathogens.

Inflammation

  • A complex response to injury/infection aiming to heal and clear pathogens.
  • Symptoms of inflammation include redness, warmth, swelling, pain, and altered function.

Fever

  • Elevated body temperature is a common response to infection.
  • Can benefit immune system function, but must be monitored for potential danger.

Adaptive Immunity

  • Relies on memory and recognition of non-self materials.
  • Key Characteristics: Specificity, Diversity, Memory, Clonality.

Antibodies and Vaccines

  • Antibodies (Immunoglobulins): Proteins produced in response to antigens with various classes (IgG, IgA, IgM, etc.).
  • Vaccines: Stimulate adaptive immunity to protect against infections without causing disease.