Pathogens and defense Knowledge-Based Study

Study notes

1. What Are Pathogens?

   A pathogen is any biological agent capable of causing disease in a host organism. The human body is constantly exposed to diverse microorganisms, but only a fraction are pathogenic.

   Pathogens generally fall into four primary categories:

   • Viruses: Non-cellular entities consisting of genetic material (DNA or RNA) enclosed in a protein capsid, requiring host cell machinery to replicate (e.g., Influenza, HIV).
   • Bacteria: Unicellular prokaryotic organisms that can damage host tissues directly or by secreting toxins (e.g., Streptococcus pneumoniae).
   • Fungi: Eukaryotic organisms that typically cause opportunistic or superficial infections (e.g., Candida albicans).
   • Parasites: Protozoa and multicellular helminths that live at the expense of the host organism (e.g., Plasmodium species).

   Innate External Defenses

   The immune system is organized into three sequential lines of defense. The first line consists of non-specific physical and chemical barriers that prevent pathogens from entering internal tissues.

   • The Skin: Provides a tough, keratinized, physical barrier. The continuous shedding of epidermal cells dislodges adhering microbes. Additionally, sebaceous glands produce sebum, which lowers skin pH (around 3 to 5) to inhibit bacterial growth.
   • Mucous Membranes: Line all internal tracts exposed to the exterior (respiratory, digestive, and urogenital). Mucus traps microorganisms, while ciliated cells in the respiratory tract sweep the mucus toward the pharynx to be swallowed or expelled.
   • Chemical Secretions: Lysozyme, an enzyme present in tears, saliva, and perspiration, actively breaks down bacterial peptidoglycan cell walls. Stomach acid (hydrochloric acid) creates a highly acidic environment (pH 1.5 to 3.5) that destroys most swallowed pathogens.

   Innate Internal Mechanisms

   When pathogens penetrate surface barriers, the body deploys rapid, non-specific cellular and molecular responses.

   Phagocytes and Natural Killer Cells

   • Neutrophils: The most abundant white blood cells, which rapidly engulf and destroy invading bacteria before dying.
   • Macrophages: Large, long-lived phagocytic cells derived from monocytes that ingest pathogens and cellular debris.
   • Natural Killer (NK) Cells: Lymphocytes that detect and destroy virus-infected host cells and tumor cells by inducing apoptosis, without prior sensitization.

   The Inflammatory Response and Antimicrobial Proteins

   • Inflammation: Triggered by tissue damage and chemical mediators such as histamine released from mast cells. Histamine causes vasodilation and increased capillary permeability, allowing phagocytes and clotting proteins to enter the affected tissue. Key symptoms include redness, heat, swelling, and pain.
   • Complement System: A cascade of plasma proteins that opsonize (coat) pathogens, recruit inflammatory cells, and form membrane attack complexes (MAC) that lyse microbial cells.
   • Interferons: Proteins secreted by virus-infected cells that stimulate neighboring cells to synthesize antiviral proteins, restricting viral spread.

   Antigen-Specific Immunity

   The adaptive immune response is highly specific, systemic, and possesses memory. It distinguishes between foreign substances using unique molecular markers called antigens.

   Humoral Immunity (B Cells)

   Humoral immunity targets pathogens present in body fluids (extracellular pathogens):

   • B lymphocytes mature in the bone marrow and display unique antigen receptors.
   • Upon binding an antigen (often with assistance from helper T cells), B cells proliferate and differentiate into plasma cells, which secrete large volumes of specific antibodies (immunoglobulins).
   • Antibodies neutralize toxins, agglutinate microbes, and mark pathogens for phagocytosis or complement activation.

   Cell-Mediated Immunity (T Cells)

   Cell-mediated immunity targets intracellular pathogens, such as viruses residing within host cells:

   • T lymphocytes mature in the thymus and recognize antigens presented on major histocompatibility complex (MHC) molecules.
   • Helper T Cells (CD4+): Secrete cytokines to activate B cells, cytotoxic T cells, and macrophages.
   • Cytotoxic T Cells (CD8+): Bind directly to infected cells and release perforin and granzymes to induce targeted apoptosis.

   Long-Term Protection

   A defining hallmark of the adaptive immune system is immunological memory.

   Primary vs. Secondary Immune Response

   • Primary Response: Occurs on first exposure to an antigen. There is a lag period of several days while specific lymphocytes undergo clonal selection and expansion, resulting in moderate antibody levels.
   • Secondary Response: Occurs upon subsequent exposure to the identical antigen. Long-lived memory B cells and memory T cells recognize the pathogen almost immediately, producing a faster, higher-affinity, and significantly larger response that typically clears the pathogen before disease symptoms manifest.

   Forms of Acquired Immunity

   • Active Immunity: Developed after the host's own immune system responds to an antigen (naturally via infection or artificially via vaccination). This produces memory cells and long-lasting protection.
   • Passive Immunity: Acquired by the direct transfer of antibodies produced by another individual (e.g., maternal antibodies crossing the placenta or injection of antivenom). This provides immediate but temporary protection, as no memory cells are formed.