Defense Against Pathogens

Overview of Immunity

  • Vertebrates have both innate (nonspecific) and adaptive (specific) immune defenses that work together to protect against pathogens by distinguishing self from non-self.

Innate Immunity

  • Innate Immunity: First line of defense, consisting of:

    • Physical barriers: Skin, mucus.

    • Antimicrobial substances: Lysozyme, antimicrobial peptides.

    • Immune cells: Phagocytic blood cells such as macrophages and neutrophils.

  • Mechanisms: Rapid response to pathogens using pattern recognition receptors (PRRs) that recognize pathogen-associated molecular patterns (PAMPs).

  • Non-adaptive features: These defenses are classified as non-adaptive because they do not improve with repeated exposure to pathogens and recognize broad classes of pathogens rather than specific antigens.

Skin and Mucosal Barriers

  • Skin: The largest organ contributing to defense with:

    • Acidic pH (3-5) inhibiting pathogen growth.

    • Secretion of lysozyme and antimicrobial peptides.

    • Normal flora preemptively outcompetes incoming pathogens.

  • Mucosal Surfaces: Lines the digestive, respiratory, and urogenital tracts, traps pathogens in mucus, and includes lysozyme in saliva and acidic secretions in the stomach.

Phagocytes in Innate Immunity

  • Phagocytic cells help eliminate pathogens:

    • Macrophages: Large cells engulfing pathogens and debris.

    • Neutrophils: Most abundant, primary responders at infection sites, effective at generating reactive oxygen species.

    • Natural Killer (NK) Cells: Attack infected or cancerous cells, inducing apoptosis.

Adaptive Immunity

  • Characterized by specificity, diversity, memory, and the ability to distinguish self from non-self:

    • Humoral immunity (B cells): Produce antibodies upon recognizing specific antigens.

    • Cell-mediated immunity (T cells): Specialize in killing infected cells or regulating other immune responses.

  • Major Characteristics of Adaptive Immunity:

    1. Specificity: Responds to particular pathogens.

    2. Diversity: Can recognize a vast number of antigens.

    3. Memory: Faster response upon re-exposure due to memory cells.

    4. Self/non-self discrimination: Distinguishes between body’s own cells and foreign cells.

Molecules in Adaptive Immunity

  • Epitope: Small part of an antigen that is recognized by the immune system.

  • Antigen: A substance that triggers an immune response, derived from pathogens or foreign bodies.

  • MHC (Major Histocompatibility Complex): Proteins on cell surfaces that present antigen fragments to T cells.

  • Antibody: Proteins produced by B cells composed of two light chains and two heavy chains; facilitates pathogen neutralization.

  • Cytokines: Signaling molecules that regulate immune responses, originating from various immune cells.

Immune Cells in Adaptive Immunity

  • Macrophage: Originates from monocytes; role in engulfing pathogens and presenting antigens.

  • Natural Killer Cells: Derived from lymphoid progenitor cells; role in targeting infected/cancerous cells.

  • Mast Cells: Found in tissues; release histamine and play a role in allergic responses.

  • B-cells: Mature in bone marrow; responsible for producing antibodies and memory cells.

  • T-cells: Mature in thymus; cytotoxic T-cells kill infected cells while helper T-cells regulate immune responses.

Distinguishing Immune Responses

  • Antigen-Presenting Cells (APCs): Capture and present antigens to T cells to activate them.

  • B-cells: When activated, differentiate into plasma cells that secrete antibodies.

  • Plasma Cells: B-cell derivatives that produce antibodies.

  • Helper T-cells: Assist in activating B-cells and cytotoxic T-cells.

  • Cytotoxic T-cells: Kill infected or abnormal cells.

Immunization and Types of Immunity

  • Active Immunity: Developed through exposure to pathogens or vaccines; triggers long-term immune response.

  • Passive Immunity: Acquired through maternal antibodies or administration of antibodies; provides temporary protection.

  • Mechanism of Immunization: Involves introducing non-pathogenic antigens to stimulate the adaptive immune response, leading to immunity.

Allergies and Autoimmune Diseases

  • Allergies: Result from inappropriate immune responses to harmless allergens, mediated by IgE.

  • Autoimmune Diseases: Occur when the immune system fails to recognize self-tissues, leading to attacks on the body’s own cells.

HIV and the Immune System

  • HIV Interaction: Targets CD4+ T cells, leading to AIDS which compromises immune response.

  • Vaccine Challenge: Difficulty lies in the virus's rapid mutation rate and its ability to evade the immune system.