Microbiology Unit VI & Chapter 18 & 19 Notes

Adaptive Specific Host Defenses

• Adaptive immunity attributes:
  • Specificity: targets specific antigens.
  • Memory: remembers past encounters for a stronger response upon re-exposure.
  • Clonality: generates clones of lymphocytes specific to an antigen.
  • Self-recognition: ability to distinguish between self and non-self.
• Humoral vs. Cellular Immunity:
  • Humoral: Involves antibodies produced by B cells that target extracellular pathogens.
  • Cellular: Involves T cells that target intracellular pathogens or abnormal cells.
• Definitions:
  • Antigen: A substance that triggers an immune response.
  • Epitope: The specific part of an antigen that an antibody or T cell receptor binds to.
  • Hapten: A small molecule that can elicit an immune response only when attached to a carrier protein.
• Good Antigen Characteristics:
  • High molecular weight.
  • Structural complexity.
  • Foreignness.
  • Accessibility.
• Antibody Structure:
  • Composed of two heavy chains and two light chains.
  • Variable regions at the tips of the chains determine specificity.
  • Constant regions determine the antibody class.
• Immunoglobulin Classes:
  • IgG: Most abundant, crosses the placenta, provides long-term immunity.
  • IgM: First antibody produced during an infection, good at complement activation.
  • IgA: Found in mucosal secretions, provides localized immunity.
  • IgE: Involved in allergic reactions and parasitic infections.
  • IgD: Function not fully understood, found on B cells.
• Antibody Functions:
  • Opsonization: Enhances phagocytosis.
  • Neutralization: Blocks pathogens from infecting cells.
  • Complement activation: Triggers the complement cascade to destroy pathogens.
  • Agglutination: Clumps pathogens together for easier clearance.
  • Antibody-dependent cell-mediated cytotoxicity (ADCC): Targets infected cells for destruction by natural killer cells.
• MHC Role in Immunity:
  • MHC molecules present antigens to T cells.
  • MHC I: Found on all nucleated cells, presents intracellular antigens to cytotoxic T cells.
  • MHC II: Found on antigen-presenting cells (APCs), presents extracellular antigens to helper T cells.
• Antigen Presenting Cells (APCs):
  • APCs engulf and process antigens, then present them to T cells.
  • Examples: dendritic cells, macrophages, B cells.
• Lymphocyte Development & Function:
  • B cells develop in the bone marrow and produce antibodies.
  • T cells develop in the thymus and mediate cellular immunity.
  • Both undergo selection processes to ensure self-tolerance.
• Central Tolerance/Self-Tolerance:
  • Elimination of self-reactive lymphocytes during development to prevent autoimmunity.
• T Lymphocyte Types:
  • Cytotoxic T cells: Kill infected or abnormal cells.
  • Helper T cells: Activate B cells and other immune cells.
  • Regulatory T cells: Suppress the immune response to prevent excessive inflammation and autoimmunity.
• Cluster of Differentiation (CD) Molecules:
  • Surface proteins on immune cells that help distinguish cell types and mediate interactions.
• Roles in Specific Immunity:
  • Plasma cells: Produce antibodies.
  • Memory cells: Provide long-term immunity.
  • Helper T cells: Coordinate immune responses.
  • Cytotoxic T cells: Kill infected cells.
  • Regulatory T cells: suppress immune responses.
  • Antigen presenting cells: Present antigens to T cells.
  • Perforin: creates pores in target cell membranes.
  • Granzymes: induce apoptosis in target cells.
  • Natural killer cells: kill infected or cancerous cells.
  • MHC I and II molecules: present antigens to T cells
  • TCR’s: T cell receptors on T cells.
  • BCR’s: B cell receptors on B cells.
• Naïve vs. Effector vs. Memory Cells:
  • Naïve cells: Have not yet encountered their specific antigen.
  • Effector cells: Actively involved in an immune response.
  • Memory cells: Long-lived cells that provide immunological memory.
• TH1 vs. TH2 Cells:
  • T_H1: Activate macrophages and cytotoxic T cells, promote cell-mediated immunity.
  • T_H2: Activate B cells and promote humoral immunity.
• T-Cell Independent vs. Dependent Activation of B Cells:
  • T-cell independent: B cell activation without T cell help, usually involves repetitive antigens.
  • T-cell dependent: B cell activation requires T cell help, results in stronger and longer-lasting immunity.
• Primary vs. Secondary Antibody Response:
  • Primary: Slower and weaker response upon initial exposure to an antigen.
  • Secondary: Faster and stronger response upon subsequent exposure due to memory cells.
• States of Immunity:
  • Active: Immunity acquired through exposure to an antigen, either naturally or artificially (vaccination).
    • Example: Infection (natural), vaccination (artificial).
  • Passive: Immunity acquired through transfer of antibodies, either naturally or artificially (antibody injection).
    • Example: Maternal antibodies (natural), antibody injection (artificial).
  • Natural: Acquired through natural processes, such as infection or maternal antibodies.
  • Artificial: Acquired through medical intervention, such as vaccination or antibody injection.
• Herd Immunity:
  • Protection of a population from infection due to a high percentage of individuals being immune.
• Antigen Preparations for Vaccines:
  • Live attenuated vaccines.
  • Inactivated vaccines.
  • Subunit vaccines.
  • Toxoid vaccines.
  • Conjugate vaccines.

Diseases of the Immune System

• Hypersensitivity Reactions:
  • Malfunction of the immune system leading to excessive or inappropriate immune responses.
• Types of Hypersensitivity Reactions:
  • Type I (Immediate): IgE-mediated, involves mast cell degranulation and histamine release.
    • Example: Allergies.
  • Type II (Cytotoxic): Antibody-mediated destruction of cells.
    • Example: Blood transfusion reactions.
  • Type III (Immune Complex): Immune complex deposition leading to inflammation.
    • Example: Serum sickness.
  • Type IV (Delayed-Type): T cell-mediated, delayed response.
    • Example: Contact dermatitis.
• ABO Blood Group System and Rh Factor:
  • Importance in blood transfusions and pregnancies to prevent immune reactions.
  • ABO: Antibodies against A or B antigens.
  • Rh: Antibodies against Rh factor (D antigen).
• Sensitization:
  • Initial exposure to an antigen that leads to an immune response upon subsequent exposure.
• Histamine:
  • Role in allergic responses, causes vasodilation, increased vascular permeability, and bronchoconstriction.
• Allergen:
  • A substance that causes an allergic reaction.
  • Categories: Inhalants, foods, injectables, contactants.
• Hypersensitivity Diagnosis:
  • Skin prick tests.
  • Blood tests.
• Hypersensitivity Treatment:
  • Avoidance.
  • Antihistamines.
  • Corticosteroids.
  • Epinephrine.
  • Immunotherapy.
• Autoimmune Disease:
  • Malfunction of the immune system leading to attack on self-antigens.
• Organ-Specific vs. Systemic Autoimmune Disease:
  • Organ-specific: targets a single organ.
    • Example: Type 1 diabetes.
  • Systemic: affects multiple organs.
    • Example: Lupus.
• Immune System in Organ Transplantation and Rejection:
  • Immune system recognizes transplanted organs as foreign and attacks them.
  • Prevention: Immunosuppressant drugs.
• Immunodeficiency:
  • Malfunction of the immune system leading to increased susceptibility to infection.
• Primary vs. Secondary Immunodeficiency:
  • Primary: Genetic defects.
    • Example: SCID.
  • Secondary: Acquired due to infection, malnutrition, or other factors.
    • Example: AIDS.