fifth lecture notes

Immunity Overview

Innate Immunity

  • Basic Characteristics

    • First line of defense against pathogens.

    • Utilizes physical barriers (skin, mucous membranes).

    • Involves innate immune responses that are not pathogen-specific.

  • Cell Types

    • Eosinophils: Break down multicellular parasites, discharge destructive enzymes.

    • Natural Killer (NK) Cells: Detect abnormal surface proteins in virus-infected or cancerous cells, leading to cell death via chemical release.

  • Lymphatic System

    • Distributes lymph, aiding in the immune response.

    • Lymph Nodes: Reservoir for macrophages and dendritic cells that engulf pathogens from lymph.

    • Interaction of dendritic cells with lymphocytes triggers adaptive immunity.

  • Antimicrobial Peptides and Proteins

    • Produced in response to pathogens, acting similarly to insect antimicrobial peptides.

    • Interferons: Proteins secreted by virus-infected cells to inhibit viral replication in nearby cells.

    • Complement System: Composed of approximately 30 proteins; activates in response to pathogens to promote cell lysis and inflammation.

Inflammatory Response

  • Characteristics of Inflammation

    • Triggered by injury or infection; results in swelling, redness, and pain.

    • Histamine: Released by mast cells, causing blood vessels to dilate and become permeable, allowing immune cells to enter tissues.

  • Phagocytic Activity

    • Activated macrophages and neutrophils release cytokines, promoting blood flow and attracting more immune cells.

    • Accumulation of pus, consisting of dead pathogens and immune cells, indicates localized immune response.

  • Systemic Response

    • Severe infection can lead to heightened systemic inflammatory responses, potentially resulting in conditions such as septic shock.

Evasion of Innate Immunity by Pathogens

  • Pathogen Mechanisms

    • Some pathogens, like Streptococcus pneumoniae, have outer capsules that hinder recognition by the immune system.

    • Opportunistic pathogens can resist lysosomal breakdown and replicate within host cells.

Adaptive Immunity

  • Characteristics

    • Unique to vertebrates; provides specific recognition and response to pathogens.

    • Involves B and T lymphocytes which mature from bone marrow-derived stem cells.

  • Antigens

    • Any substance that elicits a response from B or T cells; typically large molecules (proteins or polysaccharides).

    • Epitopes: The specific parts of antigens that are recognized by antibodies or receptors on B and T cells.

  • B and T Cell Recognition

    • B cell receptors (BCRs) recognize intact antigens; T cell receptors (TCRs) recognize antigen fragments presented by MHC molecules.

  • B Cell Activation

    • Binding of antigen to BCR leads to B cell activation, proliferation, and differentiation into plasma cells that secrete antibodies.

  • T Cell Activation

    • Requires interaction with antigen-presenting cells displaying antigen-MHC complexes; helper T cells stimulate both B cells and cytotoxic T cells.

Clonal Selection and Immunological Memory

  • Clonal Selection

    • Upon activation, B and T cells proliferate into clones of cells specific to the recognized antigen.

  • Memory Cells

    • Some cloned cells become long-lived memory cells, enabling faster and stronger responses upon re-exposure to the same antigen.

Humoral vs. Cell-Mediated Immune Responses

  • Humoral Response

    • Mediated by antibodies secreted by plasma cells; operates in bodily fluids.

    • Functions include neutralization of toxins and pathogens, opsonization, and activation of complement systems.

  • Cell-Mediated Response

    • Involves cytotoxic T cells that kill infected host cells; effective against intracellular pathogens.

Immunization

  • Provides long-term protection through immunological memory against specific pathogens due to prior exposure or vaccination.

Active vs. Passive Immunity

  • Active Immunity: Generated by the host’s immune response to infection or vaccination.

  • Passive Immunity: Provides short-term protection (e.g., antibodies transferred from mother to fetus).

Monoclonal vs. Polyclonal Antibodies

  • Monoclonal Antibodies: Identical and specific to one epitope; produced from a single B cell clone.

  • Polyclonal Antibodies: Mixture from multiple B cell clones, recognizing different epitopes.

Transfer and Rejection of Tissues

  • Tissue and organ transplants involve MHC recognition, leading to potential immune rejection.

Immune Regulation and Disorders

  • Hyperactive Responses / Allergies: Exaggerated immune responses to harmless antigens, mediated by IgE antibodies.

  • Autoimmune Diseases: Occur when the immune system erroneously targets the body's own tissues, as seen in conditions like lupus and rheumatoid arthritis.

  • Immunodeficiency: Can be congenital or acquired, leading to increased susceptibility to infections.

Pathogen Adaptations

  • Pathogens develop mechanisms like antigenic variation and latency to evade immune detection.

    • Antigenic Variation: Changes in surface proteins to avoid immune response; e.g., the influenza virus's constant mutation.

    • Latency: Ability of some viruses to enter a dormant state, avoiding immune recognition.

Conclusion

  • Understanding the immune system's innate and adaptive components is crucial for developing treatments and vaccines, and combating infectious diseases, autoimmune disorders, and immunodeficiencies.