Host-Pathogen Interactions Overview

Host-Pathogen Interactions

• The human body is constantly interacting with various microorganisms.
• Some microorganisms are normal flora/microbiota, peacefully coexisting with the host for mutual benefit.
• Other microorganisms are pathogens, capable of causing host damage via direct microbial action or inappropriate immune response.

Types of Host-Microbe Relationships

• Commensalism: One organism benefits, the other is neither harmed nor benefited.
• Parasitism: One organism benefits at the expense of the other.
• Mutualism: Both organisms benefit.

Pathogens Affecting Human Hosts

• Bacteria: Prokaryotic, single-celled organisms with a single DNA chromosome and cell walls (e.g., E. coli).
• Viruses: Smaller than bacteria, with DNA or RNA genomes; obligate parasites needing host cells to replicate (e.g., CMV, HIV).
• Protozoa: Single-celled eukaryotic organisms with organelles and simple or complex life cycles (e.g., P. falciparum, leishmania).
• Helminths: Multicellular organisms, often worms, with simple or complex life cycles (e.g., A. lumbricoides).
• Fungi: Eukaryotes with cell walls containing chitin or cellulose, and form spores (e.g., C. albicans).
• Arthropods: Organisms with jointed appendages; commonly ectoparasites (e.g., S. scabiei).
• Prions: Small proteinaceous infectious particles without nucleic acid genomes (e.g., in CJD or BSE).

Pathogen Size Comparison

• Images show pathogens and their comparative sizes (from meters to nanometers) relative to the naked eye, light microscope, and electron microscope.

Types of Transmission and Their Control

• Respiratory or salivary spreads (aerosols, saliva) are difficult to control.
• Fecal-oral spread is controllable with public health measures.
• Venereal spread is difficult to control due to social factors.
• Arthropod-borne infections and zoonoses (e.g., malaria, typhus, plague) involve vectors and vertebrate reservoirs for transmission.

Outcome of Infectious Process

• Eviction: Pathogen elimination.
• Asymptomatic carriage: Pathogen contained in the host, potentially leading to latent or carrier status.
• Clinical disease: Symptoms following incubation period (acute or chronic).

Pathogen (Invader) and Host (Defender)

• Pathogen (Invader): Bacteria, viruses, parasites, fungi causing host damage and disease.
  • Contagious potential (inoculum size, transmission, entry, adherence, tissue invasion)
  • Virulence factors and pathogenicity (direct or indirect effects).
  • Evasion mechanisms to host immunity.
  • Acquisition of drug resistance capacity.
• Host (Defender): Immune system (organs, cells, soluble factors).
  • Physical, chemical, and biological barriers.
  • Innate (general) immunity.
  • Adaptive (specific) immunity.
  • Immune suppression may lead to opportunistic infections.

Pathogen-Associated Antigens

• Antigens trigger immune responses.
• Innate immunity, antibody formation, and T-cell-mediated immunity are activated in response to antigens.
• Pathogen-associated molecular patterns (PAMPs).
  • Molecules found on pathogens that activate innate immune receptors (PRRs).
• Pathogen-associated specific antigen epitope (antigenic determinant).
  • Part of an antigen recognized by adaptive T and B cells receptors.

Organs of the Immune System

• Immune system is located in various organs.
• Primary organs for immune cell development are bone marrow and thymus.
• Secondary organs for immune responses are elsewhere (e.g., spleen).

Lymphatic Circulation

• Lymphatic system functions in fluid transport and immune responses.

Cells of the Immune System

• Key immune cells (e.g., phagocytes, lymphocytes, natural killer cells) are listed with functions.

Specific Immune Cells with Functions

• Macrophage: Presenting antigens to helper T cells, producing cytokines (chemicals involved in cellular communication).
• Neutrophil: Phagocytosis (engulfing pathogens).
• Eosinophil: Kills parasites.
• Basophil: Inflammation.
• Mast cell: Inflammation and allergies.
• Dendritic cell: Presenting antigens to T cells.

Lymphocytes

• Key role in adaptive immune responses.
• B cells: Recognize antigens via membrane-bound antibodies; produce antibodies.
• T cells: Coordinate immune responses and kill infected/abnormal cells.
• Natural killer cells (NK cells): Kill stressed/unhealthy cells.

Soluble Factors

• Complement: Direct cell lysis (breaking down cells), enhancement of lymphocyte responses.
• Cytokines: Cell-to-cell/tissue communication and various immune responses (e.g., inflammation, antibody production, cell growth/differentiation).
• Interleukins: Various immune functions (inflammation and differentiation).
• Interferons: Resistance to viral infections.
• TNFs: Inflammation and tumor cell destruction.
• Other chemicals: Various biochemical activities (e.g., antimicrobial actions).

Three Lines of Defense

• Physical, chemical, and biological barriers: Preventing pathogen entry.
• Innate immunity: General response to destroy pathogens before they establish themselves.
• Adaptive immunity: Specific response targeting and remembering particular pathogens.

Surface Barriers

• Physical barriers (intact skin).
• Chemical barriers (secretions like saliva, tears, acidic environment).
• Biological barriers (normal flora/microbiota).

Innate Immunity

• Non-specific immune mechanisms targeting pathogen elimination.
• Binding of PRRs (Toll-like receptors) to PAMPs triggers immediate defense responses.
• Key processes: fever, inflammation, phagocytosis, complement activation, natural killer cells.

Inflammation

• Response to tissue damage or infection.
• Involves chemical signals, blood vessel changes, and immune cell migration to eliminate pathogens.

Phagocytes

• Cells engulf and destroy pathogens/particles.
• Key steps include chemotaxis and adherence, ingestion, phagosome formation, phagolysosome formation and digestion, residue removal.

Complement

• Part of the innate immune system that enhances other immune responses.
• Facilitates pathogen clearance, inflammation, and cell lysis (breaking down cells).

Adaptive Immunity

• Specific and targeted response tailored to specific pathogens encountered throughout life.
• Key features: specificity, memory, and diversity.

Adaptive Immunity, cont.

• B and T cells recognize and bind to specific antigens, divide by mitosis.
• Descendent cells recognize the same antigen.
• T cells only recognize antigen presented by antigen-presenting cells (APC).
  • APC digest particles and display antigen in MHC complexes.

Cell-Mediated Immunity (Cellular Immunity)

• Processes like antigen "presenting" by macrophages to T cells activate immune responses.
• Cytotoxic T cells kill infected/abnormal cells and coordinate helper T cell activity.

Antibody-Mediated Immunity (Humoral Immunity)

• Antibodies bind to antigens, facilitating phagocytosis, complement activation/lysis, and neutralizing pathogens.

Adaptive Immunity, cont.

• Differentiated lymphocytes (B and T cells) initiate immediate responses to fight infection.
• B cells produce antibodies targeting specific antigens in bodily fluids; activate complement and phagocytosis or neutralize pathogens.
• Cytotoxic T cells kill infected or abnormal cells.
• Memory cells persist to provide a faster and stronger response to future encounters with familiar antigens.

Primary and Secondary B Cell Responses

• Primary response to first exposure produces effector and memory B cells.
• Secondary response triggered by subsequent exposures is stronger and faster due to memory cells.

Innate and Adaptive Immunity Comparison

• Innate: Immediate response, non-specific.
• Adaptive: Delayed response, specific.

Nature of Invader Determines Host Immune Response

• The type of pathogen dictates how the body responds immunologically. Different pathogens trigger different immune responses.

Pathogen Evasion Mechanisms

• Pathogens employ various mechanisms to evade the host's immune responses (e.g., antigenic variation, complement inhibition, evasion of phagocytosis).

Environment: Source & Reservoir

• Source: Site where a pathogen is transmitted.
• Reservoir: Site where pathogens multiply.
• Examples like humans, animals, food, water.
  • Yellow fever with monkeys as a reservoir, mosquitos as a source.