Overview of Immune System

Overview of Immune System

• The immune system's main function is to prevent or limit infections, fungi, and parasites.
• The immune system has two arms: Innate Immunity and Acquired Immunity.

Innate Immunity

• Present since birth.
• Less efficient immediately after infection.
• Non-specific, protecting against all microorganisms at all times.
• No immunological memory, offering no long-term protection.
• Interacts with acquired immunity through antigen presentation.
• Main cells include neutrophils, monocytes, macrophages, and natural killer (NK) cells.

Acquired Immunity

• Develops throughout a person's life after exposure to pathogens.
• More effective than innate immunity.
• Specific, targeting certain pathogens.
• Improves with repeated exposure.
• Develops immunological memory, enabling long-term protection.
• Interacts with innate immunity through opsonization.
• Main cells include B and T lymphocytes.

Cells of the Immune System

• Stem cells differentiate into lymphoid and myeloid progenitor cells.
• Lymphocytes include B cells, T cells, and natural killer (NK) cells.
• Myeloid progenitors differentiate into granulocytes (neutrophils, eosinophils, basophils, and mast cells) and monocytes, which develop into macrophages (e.g., kupffer cells in the liver and alveolar macrophages in the lungs).
• Dendritic cells are also part of the immune system.

Mechanisms of Innate Immunity

• Mechanical barriers and surface secretions: Intact skin, mucous membranes, cilia of the respiratory tract, blinking, sneezing, and coughing.
• Normal bacterial flora: Organisms that compete for essential nutrients.
• Humoral defense mechanisms: Lysozyme, complement, acute-phase proteins, and cytokines.
• Cellular defense mechanisms: Phagocytic cells (neutrophils, macrophages, eosinophils) and natural killer cells.
• Inflammation.

Mechanisms of Acquired Immunity

• T-cell mediated immunity: Consists of helper T cells and cytotoxic T cells.
• Humoral immune system: Consists of B lymphocytes that differentiate into plasma cells and produce immunoglobulins.

Lymphoid Organs

• Organized tissues where immune functions occur.
• Lymphoid cells interact with other cells.
• Maturation of immune cells.
• Starting of acquired immune response.
• Primary lymphoid organs: Bone marrow (origin of all immune cells and B cell maturation), and thymus (T cell maturation).
• Secondary lymphoid organs: Lymph nodes, spleen, and mucosa-associated lymphoid tissue (MALT).

Role of Cells in the Immune System

• Granulocytes: Neutrophils (phagocytes, most numerous, form pus), eosinophils (important in allergies and against helminthes), mast cells (in tissues, release mediators in allergies/inflammation), and basophils (similar to mast cells).

• Monocytes/macrophages: Monocytes in blood become macrophages in tissues; phagocytosis and antigen presentation (e.g., Kupffer cells in the liver, alveolar macrophages in lungs).

• Lymphocytes:

  • T lymphocytes (originate in bone marrow, mature in thymus; 75% peripheral blood lymphocytes, two types including T helper cells, whose main function is cytokine secretion, and T cytotoxic cells, whose main function is killing target cells).
  • B lymphocytes (originate and mature in bone marrow, 10% peripheral blood lymphocytes; differentiate into plasma cells that produce antibodies).

• Natural Killer Cells: Large granular lymphocytes; non-T, non-B; kill virus-infected cells.

Antigen-Antibody Reactions

• Antigen-antibody reactions are highly specific.

• An Antibody will react only with the antigen that induced it, or a closely related antigen.

• Antibodies (immunoglobulins) are glycoproteins which bind specifically to antigens inducing their formation.

• The immune system doesn't recognize the whole antigen molecule but limited parts called epitopes (4-5 amino acids).

Diagnostic Tests

• Results are expressed as a titer (highest specimen dilution with a positive reaction).
• Antibody-based tests (serological tests) diagnose infections, autoimmune diseases, and type blood.
• Types of diagnostic tests: Agglutination (particles clumping), Precipitation (soluble antigens).

Hypersensitivity

• Type I: Immediate or anaphylactic; involves IgE, mast cells, basophils; reactions occur in minutes (e.g., allergic reactions).
• Type II: Cytotoxic; antibodies (IgM, IgG) and complement cause cell lysis; reactions occur in minutes to hours (e.g., blood transfusion reactions).
• Type III: Immune complex; antigen-antibody complexes deposit in tissues; reactions occur in 3-10 hours (e.g., serum sickness).
• Type IV: Delayed-type; T cells cause tissue damage; reactions occur in days (e.g., contact dermatitis).

Autoimmune Diseases

• Autoimmune disease is a disorder where the body's immune system attacks its own tissues.
• Proposed mechanisms for autoimmunity include forbidden clones, altered antigens, sequestered antigens, immunological deficiency theory, and genetic influences.
• Types of autoimmune diseases; Organ-specific (affecting a particular organ, e.g., type 1 diabetes, Hashimoto's disease and Grave's disease) and systematic, affecting many organs/tissues (e.g., lupus erythematosus).

Introduction to Microorganisms

• Microorganisms include bacteria, viruses, fungi, protozoa, and helminthes.
• Bacteria: Prokaryotes (no nucleus and membrane-bound organelles) with various shapes (cocci, bacilli, spirilla).
• Viruses: Acellular; composed of nucleic acid (DNA or RNA) and a protein coat; replicate inside host cells.
• Fungi: Eukaryotic; have a cell wall primarily composed of chitin; reproduce by budding or spore formation.

Structure of Bacteria

• Shape: Cocci (spherical), bacilli (rod-shaped), spirilla (spiral-shaped).
• Arrangement: Diplococci (pairs), streptococci (chains), staphylococci (clusters).

External Structures of a Bacterial Cell Wall

• Glycocalyx (capsule or slime layer): Helps prevent phagocytosis, adhere to surfaces (e.g., dental surfaces).
• Flagella: Motility.
• Axial filaments: Motility (in spirochetes).
• Fimbriae and Pili: Attachment.

Bacterial Cell Wall

• The cell walls of bacteria (peptidoglycan) is responsible for the shape, prevents rupture, and staining affinity of the bacteria.

Gram-Positive Cell Wall

• Many layers of peptidoglycan.
• Teichoic acids are present.
• Negative charge from phosphate groups.

Gram-Negative Cell Wall

• One or few layers of peptidoglycan.
• Outer membrane (lipopolysaccharides (LPS), lipoproteins, and phospholipids) containing porins.
• LPS has an important role in evading phagocytosis.

Acid-Fast Cell Wall

• High concentrations of mycolic acid.
• Mycolic acid prevents uptake of dyes.

Internal Bacterial Structures

• The cytoplasmic membrane (or inner membrane):phospholipid bilayer with proteins; involved in transport, energy production and synthesis of cell wall.
• Cytoplasm.
• Ribosomes: Protein synthesis (70S).
• Nucleoid: DNA.
• Inclusion bodies: Storage of nutrients.
• Endospores: Dormant, resistant structures formed in adverse conditions (gram-positive bacteria) that are highly resistant.

Bacterial Growth and Physiology

• Growth: Increase in bacterial numbers and size (typically by binary fission).
• Generation time: Time required for a population to double.
• Physical requirements: Temperature, pH, osmotic pressure.
• Chemical requirements: Carbon source, nitrogen, sulfur, phosphorus source, and organic growth factors

Bacterial Pathogenesis

• Infection: Invasion/colonization of the body.
• Disease: Abnormal state in which a body part/system is not working properly.
• Pathogenic Bacteria: Bacteria capable of causing disease.
• Non-pathogenic Bacteria: Do not cause disease (part of normal flora).
• Opportunistic Pathogens: Cause disease under abnormal (e.g., immunocompromised) conditions.

Mechanisms of Pathogenicity

• Adherence
• Invasion
• Antiphagocytic factors
• Biofilm formation
• Toxin production

Exotoxins and Endotoxins

• Exotoxins: Proteins secreted by bacteria, highly toxic, can be inactivated to form toxoids, and often enzymatic action(secreted from living cells, highly toxic, easily inactivated by heat, can be converted to toxoids.)

• Endotoxins: Lipopolysaccharides (LPS) released from the cell walls of gram-negative bacteria during cell lysis, moderately toxic, and not easily inactivated by heat.

General Mycology

• Fungi: Eukaryotic microbes with cell walls made of chitin.
• Types of fungi: Yeasts (single-celled), molds (filamentous).
• Features: Nucleus, mitochondria, sterol, and cell wall components differentiate fungi from bacteria.

Structure and General Features of Fungi

• Yeasts: Reproduce by budding.
• Molds: Form hyphae (filaments) that create a mycelium.
• Some fungi are dimorphic; existing as molds at one temperature and yeasts at another.

Fungal Classification/Pathogenesis and Clinical Problems

• Classify by morphological features and clinical presentation.
• Morphological classification: Molds, yeasts, and dimorphic (change forms).
• Fungal infections: Superficial, cutaneous, subcutaneous or deep/systemic.
• Fungal allergies occur due to fungal spores.
• Mycotoxicosis: Caused by potent fungal toxins (e.g., aflatoxin).

Sterilization and Disinfection

• Decontamination: Reducing microorganisms to safe levels.
• Cleaning: Removing visible contaminants.
• **Disinfection:**Removing most microorganisms except for bacterial spores.
• Sterilization: Complete removal of all microorganisms, including spores.

Infection Control

• Infection control (IC): Procedures to prevent, detect, and reduce risks of infections.

• Who is concerned?: Healthcare workers (HCWs) including doctors, nurses, technicians and anyone who has contact with patients.

• High risk areas: ICUs, operating rooms (ORs), and locations where there is high risk of exposure to infectious agents.

• Hand hygiene: The most effective way to prevent transmission.

• Standard precautions: Precautions used for all patients regardless of diagnosis.

• Transmission based precautions: Precautions used for certain pathogens/infections.

• Isolation precautions: Used to prevent transmission; segregating organism, not the patient.

• Airborne, droplet, contact: specific types of isolation precautions.

• Waste management: Infectious waste disposal procedures.

• Sharp disposal: Procedures to prevent needle stick injuries; use disposable sharps containers.

Bacterial Classification

• Gram-positive bacteria: Cell wall with a relatively thick layer of peptidoglycan, retains crystal violet dye in a Gram stain procedure.

• Gram-negative bacteria: Outer membrane outside of the peptidoglycan layer, does not stain with crystal violet dye.

Bacterial Diseases/ Infections

• Streptococcus:

  • Group A: Important cause of pharyngitis, impetigo, and other infections; associated with rheumatic fever and glomerulonephritis.

• Group B: Leading cause of neonatal sepsis and meningitis.

  • Viridans: Important cause of bacterial endocarditis.

• Osteomyelitis: Infection in the bone.

• Septic arthritis: Infection in a joint.

Viral Structure

• Virion: Complete, fully developed infectious viral particle.

• Capsid: Protein coat protecting the nucleic acid. (composed of protein subunits called capsomeres).

• Nucleic Acid: DNA or RNA genome

• Enveloped viruses : have an extra lipid membrane derived from the host cell membrane.

Viral Classification

• Classification based on Morphology ( presence or absence of envelope) , Size of the virion and the type of host or host structures infected.

Viral Diseases

• Poliomyelitis: Infection to the nervous system by the poliovirus, leading to paralysis.
• Meningitis: Inflammation of the meninges (membranes surrounding the brain and spinal cord) often due to infection (Bacterial or viral).

Antimicrobial Drugs and vaccination

• Antimicrobials: Drugs to eliminate or reduce bacterial infection.

• Vaccination: Immune response by exposure to a weakened/inactivated form of the disease-causing agent to prevent possible infection.

Test your knowledge on the immune system with this quiz covering innate and acquired immunity. Learn about the functions, cells involved, and how these two arms of the immune system interact. Perfect for students looking to understand the basics of immunology.