Immunology Quiz on Immune Responses

Immunology

• The study of how the body ("host") recognizes self from nonself ("pathogens")
• Goal: Eliminate foreign matter without damaging the host

Types of Pathogens

• Viruses
• Bacteria
• Fungi
• Parasites

Lymphoid Organs

• Primary Lymphoid Organs - Responsible for the maturation of lymphocytes
  • Bone marrow: Site of B cell maturation
  • Thymus: Site of T cell maturation
• Secondary Lymphoid Organs - Activation of lymphocytes during a specific immune response
  • Lymph nodes (LN)
  • Spleen
  • Tonsils/adenoids
  • Lymphocyte accumulations in the gut, respiratory, genital, and urinary tracts (e.g., GALT, BALT)

The Immune System

• Two main parts:
  • Innate Immune Response
  • Adaptive Immune Response

Innate Immunity

• Present at birth
• Acts immediately
• Does not require activation
• Nonspecific
• Recognizes carbohydrates and lipids on foreign cells
• Leukocytes circulate in the blood

Adaptive Immunity

• Acquired through exposure to pathogens
• Takes time to develop
• Requires activation
• Specific - forms memory cells
• Recognizes proteins on foreign cells
• Lymphocytes present in secondary lymphoid organs

Components of Innate Immunity

• Physical Barriers:
  • Skin
  • Epithelium
  • Mucous membranes
• Chemical Barriers:
  • Lysozyme
  • Sweat
  • Stomach acid
• Cellular Components (Leukocytes):
  • Polymorphonuclear Granulocytes (PMNs):
    • Neutrophils: Most numerous, secrete bleach and peroxide, avid phagocytes, elevated in bacterial infections.
    • Basophils: Secrete histamine, related to mast cells (in skin and mucus membranes).
    • Eosinophils: Fight parasite infections, elevated in allergies.
  • Macrophages: Derived from monocytes, found where epithelium meets external environment (skin, gastrointestinal and respiratory tract), engulf particles by phagocytosis, act as Antigen-presenting cells (APCs).
  • Dendritic cells: Macrophage-like cells, act as APCs.
• Non-cellular Components:
  • Complement: A group of plasma proteins that kill microbes extracellularly, synthesized in the liver and circulate in the blood.
  • Cytokines: Protein messengers released by cells of the immune system.
• Inflammation: Local response to infection or injury

Physical and Chemical Barriers

• Skin: Few microorganisms can penetrate intact skin.
• Epithelium (Gastrointestinal and Respiratory System):
  • Mucous membranes: Sticky, traps and limits bacterial adhesion to epithelium.
  • Hair and cilia: Trap and sweep away particulate matter.
• Lysozyme: Enzyme that lyses bacterial cell walls, found in tears, saliva, milk, and mucus.

Cellular Components

• Leukocytes:
  • Neutrophils: Most numerous, secrete bleach and peroxide, avid phagocytes, elevated in bacterial infections.
  • Basophils: Secrete histamine, related to mast cells (in skin and mucus membranes).
  • Eosinophils: Fight parasite infections, elevated in allergies.
  • Macrophages: Derived from monocytes, found where epithelium meets external environment (skin, gastrointestinal and respiratory tract), engulf particles by phagocytosis, act as Antigen-presenting cells (APCs).

Non-Cellular Components

• Complement: Family of plasma proteins (30+ proteins)
  • Triggers extracellular killing of microbes
  • Synthesized in the liver and circulates in the blood
  • Activated by infection or damage
  • Forms the membrane attack complex (MAC) that causes cell lysis
• Cytokines: Protein messengers released by cells of the immune system
  • Facilitate communication within the immune system
  • Often act in cascades, with one cytokine stimulating the release of another
  • Play a role in both innate and adaptive immune responses
  • Types: Interleukins (IL-1, IL-2, etc.), Interferons (IFN), Chemokines, Lymphokines, Tumor Necrosis Factor (TNF)

Inflammatory Response

• Nonspecific immune response to invasion by pathogen (similar response to trauma, cold, heat injury)
• Steps:
  1. Entry of bacteria into tissue, causing tissue injury and chemical release.
  2. Vasodilation (increased blood flow) in the infected area (warmth, redness, swelling).
  3. Increased permeability of capillaries and venules.
  4. Chemotaxis: Movement of leukocytes into interstitial fluid and tissue.
  5. Destruction of bacteria by phagocytosis or other mechanisms.

Adaptive Immune Response

• Two types:
  • Cell-mediated Immunity: Carried out by activated T cells.
  • Humoral-mediated Immunity: Carried out by activated B cells.

Components of Adaptive Immunity

• Cellular Components:
  • B lymphocytes (B cells): CD19, CD20, responsible for humoral-mediated immunity.
  • T lymphocytes (T cells): Responsible for cell-mediated immunity.
    • Cytotoxic T cells (Tc, CD8): Kill infected cells directly.
    • Helper T cells (Th, CD4): Coordinate immune response.
• Antigen-presenting cells (APCs): Required for the activation of adaptive immune cells.

B Cell Receptors

• Antibodies on the surface of B cells
• Bind to specific antigens like a lock and key

T Cell Receptors

• Do not directly recognize antigens
• Recognize antigens presented on major histocompatibility complexes (MHCs)
• MHCs are unique to each person and act as genetic "identity tags" of self.
• Two classes of MHCs:
  • Class I MHC: Found on all nucleated cells (except red blood cells).
  • Class II MHC: Found only on the surface of macrophages, dendritic cells, and B cells.
• T cell receptors can only recognize an antigen if it is bound to an MHC.
  • Cytotoxic T Cells (CD8): Bind to Class I MHC.
  • Helper T Cells (CD4): Bind to Class II MHC.

Human Leukocyte Antigen (HLA) Complexes

• Human version of the major histocompatibility complex
• Encoded by a cluster of genes on chromosome 6
• Used to match for tissue transplantation to prevent rejection
• Certain HLA types are associated with specific autoimmune diseases
• Some HLA-mediated diseases may predispose to cancer

Details of Adaptive Immunity

• Helper T Cells (Th): Coordinate the immune response to a specific pathogen, do not directly attack, but begin and coordinate the response.
  • Enhance inflammation
  • Stimulate cytotoxic T cells
  • Stimulate B Cells
  • Activated by antigen-presenting cells, producing IL-2.
  • Two types: Th1 (cell-mediated) and Th2 (humoral), typically in balance.
  • IL-2 causes helper T cells to proliferate and differentiate into:
    • Memory helper T cells (remember the specific antigen)
    • Effector helper T cells (coordinate immune response)
• Cytotoxic T Cells (Tc): Kill infected cells directly.
  • Activated by antigen-presenting cells.
  • IL-2 from helper T cells causes Tc proliferation and differentiation into:
    • Memory cytotoxic T cells (remember the specific antigen)
    • Effector cytotoxic T cells (migrate to the site of infection and kill pathogens or infected cells).
• B cells:
  • Sensitized to antigen either by phagocytosis or antigen-presenting cells.
  • Receive signals from helper T cells via IL-4, causing proliferation and differentiation into:
    • Memory B cells (remember the specific antigen)
    • Plasma cells (remain in secondary lymphoid organs and secrete antibodies).

How Do Antibodies Destroy Pathogens?

• Antibodies are secreted by plasma cells into lymph and blood.
• Bind to pathogens at the site of infection or in circulation.
• Destroy pathogens in three ways:
  • Neutralization: Prevents virus or bacteria from binding to host cells.
  • Opsonization: Tags a pathogen for destruction by phagocytes.
  • Complement activation: Destroys pathogen by forming pores in the cell membrane.

Immunoglobulins

• Family of proteins that includes B cell receptors and plasma cell antibodies.
• Composed of four interlinked chains:
  • Two heavy chains (long chains): Stem is distinct for each Ig type.
  • Two light chains (short chains): Contain constant and variable regions.

Immunoglobulin Classes

• "MADGE"
  • IgM: First antibody produced after new infection, transient.
  • IgG: Most abundant, crosses placenta, long-term memory, active in secondary response.
  • IgE: Involved in allergic reactions and parasite infections (found on mast cells and basophils).
  • IgA: Mucosal immunity, found where the body interacts with the external environment (gastrointestinal, respiratory, genitourinary systems), secreted in breast milk.
  • IgD: Found in blood and lymph, function unknown.

Vaccination to Induce Immunity

• Contains noninfectious antigens from known pathogens to generate a mild primary immune response.
• Leads to immune memory (memory B cells, IgG).
• When exposed to the pathogen, the immune system mounts a quick and robust response.

Immunologic Testing

• Polymerase Chain Reaction (PCR): Detects genetic material from an organism or pathogen.

Test your knowledge on the fundamentals of immunology, including the recognition of pathogens and the functions of lymphoid organs. Explore the differences between innate and adaptive immune responses. This quiz covers key concepts essential for understanding the immune system.