MICROBIO 1.3

Host-Pathogen Interactions Pt. I

• The first part of the course is an introduction to the immune system.
• The previous class covered microbiota and disease establishment.
• The current class focusses on how the body fights infections by describing the immune system.

Immunity

• Innate immunity and adaptive immunity are the two components of the immune system.
• Innate immunity involves physical barriers, chemical barriers, and cellular defenses.
• Adaptive immunity features active and passive immunity acquired through natural means, vaccination, maternal transfer, or artificial means.

Class Objectives

• Differentiate between innate and adaptive immunity.
• Describe the components and functions of the innate immune system.
• Identify immune cells based on their roles and pictures.
• Explain how the innate system activates the adaptive system.
• Understand the different types of lymphocytes and their activation/roles.
• Define antibodies and immunological memory.
• Differentiate between MHC class I and MHC class II.

Big Picture

• Antigens are foreign substances triggering an immune reaction.
• Exposure to antigens results in an immune response or tolerance.
• Innate and adaptive immunity work together in response to pathogens.

Innate vs. Adaptive Immunity

• Innate immunity is rapid but non-specific.
• Adaptive immunity is slower but highly specific.
• Innate immune response to a first exposure is more significant than a second.
• Adaptive immune response to a second exposure is much more significant than a first.

Cells of Innate & Adaptive Immune System

• Innate and adaptive immune cells share a common origin.
• Innate immune cells include myeloid stem cells, such as neutrophils, macrophages, basophils, eosinophils, monocytes among others.
• Adaptive immune cells include lymphoid stem cells, such as lymphocytes, B-cells, T-cells.

Innate Immune System

• Early, generalized first-line defense.
• Recognizes microbes via Pattern Recognition Receptors (PRRs) that bind to Pathogen-Associated Molecular Patterns (PAMPs).
• Involved in general responses to pathogens.
• Includes phagocytes, dendritic cells, natural killer cells, and pro-inflammatory cells.

Components of the Innate Immune System: Cellular

• Skin, mucous membranes, and mechanical defenses (coughing, sneezing).
• Phagocytic cells (neutrophils, macrophages).
• Pro-inflammatory cells (macrophages, mast cells, eosinophils, basophils).
• Antigen-Presenting Cells (APCs) initiate adaptive responses.
• Natural Killer (NK) cells kill infected cells.
• Neutrophil extracellular traps (NETs).

Components of the Innate Immune System: Humoral

• Body fluids like bile, acids, tears, mucous- these use antimicrobial peptides to fight pathogens.
• Complement system enhances phagocytosis and induces inflammation.
• Cytokines- proteins released by cells signaling for reactions (pro-inflammatory & anti-inflammatory).
• Chemokines- cytokines that attract leukocytes (inflammatory response).

Complement - "Complements Antibodies"

• Complement system makes invading micro-organisms more susceptible via phagocytosis (opsonization) or lysis
• Promotes inflammatory response
• The complement pathway produces substances that attract white blood cells
• Complement triggers either classical (antigen-antibody complexes), alternative (bacterial LPS), or lectin (mannose sugars) pathways.
• Can result in the formation of the membrane attack complex (MAC) that causes bacterial lysis.
• LPS (lipopolysaccharide) can block MAC, preventing Gram-negative bacteria from being killed.

Consequences of Recognizing Antigen

• Increased antimicrobial peptide production
• Secretion of inflammatory mediators.
• Activation of complement
• Activation of clotting cascade
• Chemotactic attraction of cells.
• Acute phase response.
• Inflammation.

Inflammation

• Characterized by redness, heat, swelling, and pain (rubor, calor, tumor, dolor).
• Leads to a loss of function.

Adaptive Immune Response

Lymphocytes

• CD8 cytotoxic T cells kill infected/damaged cells.
• CD4 helper T cells assist cytotoxic T and B cells.
• B cells produce antibodies

Lymphocyte Development, Activation, & Antigen Presentation

• T and B cells go through development to recognize self from non-self.
• Activated T cells divide and proliferate.
• Helper T cells secrete cytokines to activate other cells.
• Cytotoxic T cells kill cells bearing antigens.
• B cells differentiate into plasma cells secreting antibodies.

Immunoglobulins (Antibodies)

• Immunoglobulins (IgA, IgD, IgE, IgG, IgM) are glycoprotein molecules secreted by plasma cells.
• They have various structural and functional roles, affecting pathogens by agglutination, neutralization, opsonization, complement activation.

Initiation of Adaptive Immune Responses

• Microbial antigens trigger dendritic cells to take up and process antigens and present on MHC class II molecules.
• They activate helper T cells through MHC class II antigen presentation.
• Activated helper T cells activate B cells to produce antibodies.

Major Histocompatibility Complex (MHC)

• Glycoproteins present antigens for T cell activation.
• MHC class I presents endogenous antigens on all nucleated cells to cytotoxic T cells.
• MHC class II presents exogenous antigens on antigen-presenting cells to helper T cells.

Immunological Memory

• Provides immunity through faster, stronger responses to repeat exposures.
• Subsequent encounters with the pathogen result in a rapid and enhanced response.

Test Your Knowledge (Various)

• Various questions about the immune system and immune cells from previous lectures/slides.

Also A Scientist

• Brigitte Askonas: A scientist who played a significant role in immunology.
• Askonas research on B cells and macrophages.

This quiz focuses on the introduction to the immune system, detailing both innate and adaptive immunity. You will explore the components, functions, and roles of immune cells and how they interact in the body's defense against infections. Test your understanding of key concepts such as antibodies and immunological memory.