Immunity and Defense

Immunity and Defense

Chapter Overview

  • Copyright: © 2016 by Elsevier, Inc.

Anatomical Organization of the Immune System

  • Components:
    • Skin and Mucous Membranes
    • Spleen
    • Lymphatic System
    • Lymph Nodes
    • Mucosa-Associated Lymphatic Tissue (MALT)
    • Tonsils
    • Peyer's Patches
    • Thymus
    • Red Bone Marrow

The Immune System

  • Definition: The security system of the body that recognizes foreign material and protects against anything that is not part of the body.
  • Types of Immune Response:
    • Innate Immune System:
    • First line of defense: External Barriers (e.g., skin, mucous membranes).
    • Second line of defense: Cellular and Chemical Components (e.g., inflammation).
    • Adaptive Immune System:
    • Third line of defense: Target Specific Pathogens (e.g., antibody production).

First Line of Defense

  • Skin:
    • Acts as a physical barrier.
    • Contains resident microorganisms.
    • Has an acidic pH and the fatty acid content of sweat.
  • Mucous Membranes:
    • Contain cilia and mucus in the respiratory system.
    • The acidity of the stomach acts as a defense.
    • Fluids such as tears, saliva, and urine contribute to protection.

Internal Protection

  • Spleen:
    • White pulp has immunological functions involving phagocytic cells reacting to antigens in the bloodstream.
    • Macrophages in red pulp remove worn, damaged blood cells.
  • Lymphatic System:
    • Responsible for collecting and returning excess interstitial tissue fluid to the cardiovascular system.
  • Lymph Nodes:
    • Small structures responsible for lymph filtration as lymph travels back to systemic circulation.
    • Lymph from specific areas of the body always passes through the same node(s) aiding in the determination of the location of inflammation, infection, or tumor.

Lymph Nodes

  • Distribution:
    • Submandibular (caudal to the mandible)
    • Prescapular (cranial to the shoulder)
    • Axillary (where the front limb joins the thorax)
    • Inguinal (near the groin)
    • Popliteal (caudal aspect of the hamstring muscle)

Mucosa-Associated Lymphatic Tissue (MALT)

  • Definition: Clusters of lymphoid tissue located near mucosal surfaces (not encapsulated like lymph nodes).
  • Components:
    • CALT: Conjunctiva-associated Lymphatic Tissue.
    • NALT: Nasopharynx-associated Lymphatic Tissue.
    • GALT: Gut-associated Lymphatic Tissue.
  • Function: Identifies antigens and mounts immune response.

Tonsils

  • Part of MALT: Present at the beginning of lymph draining system.
  • Absence of Capsule: Tonsils lack a capsule and are found in epithelial tissue.
  • Location: Pharynx, larynx, urinary and reproductive tracts.
  • Function: Destroy foreign material before it enters the body and causes disease.

Peyer's Patches

  • Definition: Aggregations of lymphoid tissue located in the small intestine.
  • Presence: Particularly abundant in cattle, sheep, pigs, horses, and dogs; mostly found in the lining of the ileum and a smaller percentage in the jejunum.

Thymus

  • Location: Found in the mediastinum of young animals.
  • Function: Site where T lymphocytes mature, after which they migrate to other lymphoid tissues and to the blood.
  • Role of T Lymphocytes: Programmed to fight specific antigens and produced throughout the life of the animal.

Red Bone Marrow

  • Primary Function: Responsible for the production of all white blood cells.

Functional Organization of the Immune System

Innate Immune System
  • External Innate Immunity:
    • Involves skin and mucous membranes.
    • Physical secretion of tears, saliva, and nasal discharges.
  • Internal Innate Defense:
    • Acts as the second line of defense through acute inflammation.
    • Fever:
    • Definition: Elevated body temperature as a systemic inflammatory response causing chemical mediators to be carried throughout the body.
    • Function: Creates an environment exceeding the optimum temperature for pathogen growth; a temperature exceeding 104°F can lead to protein denaturation.
Phagocytosis
  • Definition: The process through which certain cells ingest and destroy pathogens.
  • Types of Cells: Neutrophils, monocytes, macrophages, dendritic cells.
  • Mechanism: Cells contain receptors to differentiate "self" from "non-self" using PAMP (pathogen-associated molecular patterns) receptors and complement receptors.
  • Steps:
    1. Activation and chemotaxis (movement toward invader).
    2. Attachment to invader.
    3. Ingestion of the invader.
    4. Destruction of the invader.
    5. Exocytosis (release of benign pieces of invader).
The Complement System
  • Definition: A group of over 30 plasma proteins, primarily inactive proteolytic enzymes produced predominantly in the liver.
  • Identification: Identified by the letter “C” followed by a number, always present in plasma in inactive form.
  • Activation: Becomes active in the presence of antigen or an antibody attached to an antigen.
  • Functions:
    • Trigger inflammation.
    • Alter microbial cell membranes to increase visibility to macrophages (opsonization).
Cytokines
  • Also Known As: Cell movers.
  • Function: Communicators within the innate immune system that attract immune cells to sites of action.
  • Types:
    1. Autocrine: Act on the cell which secreted them.
    2. Paracrine: Act on neighboring cells.
    3. Endocrine: Travel away from the secreting cell.
  • Role: Attract immune cells to specific sites, inhibit molecules, enhance immune processes, and are involved in hematopoiesis.
  • Examples:
    • Interleukins: control leukocyte growth and activation.
    • Interferons: produced in response to viral infections.
    • Chemokines: stimulate leukocyte movement to injury sites.
Natural Killer (NK) Cells
  • Location: Found in blood and lymph; involved in both innate and adaptive immune systems.
  • Function: Bind to target cells inducing apoptosis without ingesting them.
  • Types of Receptors:
    • Killer Inhibitory Receptor (KIR): Binds to MHC-1 molecules on normal cells to avoid killing healthy cells.
    • Killer-Activating Receptor (KAR): Recognizes virus-infected and tumor cells with altered MHC-1, leading to cell death via perforins and proteolytic enzymes.
Interferons
  • Definition: Proteins produced in response to the presence of viruses, bacteria, cancer, and other foreign invaders.
  • Mechanism: bind to receptors on surrounding non-infected cells to produce inactive anti-viral particles (AVPs); upon virus entry, AVPs are activated inhibiting viral replication.

Adaptive Immune System

  • Definition: Also known as acquired immunity, it is characterized by the ability of B lymphocytes and T lymphocytes to remember and respond to specific pathogens.
B Lymphocytes
  • Origin: Formed in the bone marrow.
  • Function: Programmed to secrete specific antibodies (immunoglobulins); migrate to lymph nodes and spleen.
T Lymphocytes
  • Precursor Cells: Thymocytes originating in red bone marrow, migrating to thymus for maturation.
  • Role: Coordinate cell-mediated immunity and activate B cells.
Memory Cells
  • Definition: Clones of T and B cells activated during an immune response that remain in lymph nodes or circulate in blood.
  • Function: When exposed to the original antigen, they activate a stronger, quicker immune response.
Humoral and Cell-Mediated Immunity
  • Humoral Immunity:
    • Triggered by extracellular pathogens, leading to the production of antibodies by B cells/plasma cells.
  • Cell-Mediated Immunity:
    • Provides immunity against intracellular pathogens without the needs for antibody production, through direct attachment of T cells to antigen markers.
Immunoglobulins
  • Types:
    • IgM: Largest antibody, temporary (disappears within 2-3 weeks of initial infection).
    • IgG: Smallest and most common, indicates chronic infections, provides passive immunity to the fetus.
    • IgA: Protects body surfaces (intestines and lungs).
    • IgE: Binds allergens and triggers histamine release, protects against parasitic infections.
    • IgD: Activates basophils and mast cells.
Active and Passive Immunity
  • Active Immunity:
    • Results from an active immune process through natural infection or vaccination (live attenuated or killed virus).
  • Passive Immunity:
    • Involves receiving antibodies from an external source (e.g., maternal antibodies through colostrum).
Factors Determining Likelihood of Disease
  • Factors include:
    • Exposure to pathogens
    • Mode of infection/transmission
    • Virulence (strength of the pathogen)
    • Immune system strength
    • Acquired resistance through exposure or vaccination.
    • Species resistance to specific diseases.
Hypersensitivity Reactions
  • Type I: Immediate hypersensitivity (e.g., anaphylaxis); animal sensitized by IgE antibodies experiences severe reactions on second exposure to the same antigen.
  • Type II: Immune system causes disease (e.g., immune-mediated hemolytic anemia).
  • Type III: Immune complexes can cause tissue damage through complement activation (e.g., chronic lupus).
  • Type IV: Delayed cell-mediated reactions (e.g., systemic inflammatory response syndrome due to severe infection).

Key Terms

  • Definitions:

    • Interferon
    • Macrophage
    • Memory cell
    • Microbe
    • Passive immunity
    • Pathogen
    • Pathogenicity
    • Phagocyte
    • Phagocytosis
    • Virulence
    • Virulence factor
    • Active immunity
    • Adaptive immunity
    • Antibody
    • Antigen
    • Apoptosis
    • Complement
    • Cytokine
    • Disease
    • Fever
    • Infection
    • Inflammation
    • Innate immunity

  • Copyright: © 2016 by Elsevier, Inc.