BIOL 2228_Imunity_chapt22revised

Chapter 22 Immunity: The Body's Defenses

22.1 Overview of Diseases Caused by Infectious Agents

  • Learning Objectives:

    1. Compare and contrast the five major classes of infectious agents.

    2. Describe prions, and name a disease they cause.

Overview of Infectious Agents

  • Infectious agents can damage or kill a host.

    • Pathogenic agents = disease-causing.

    • Five major categories:

      • Bacteria

      • Viruses

      • Fungi

      • Protozoans

      • Multicellular parasites

22.1.1 Bacteria

  • Characteristics:

    • Single-celled prokaryotes (1 to 2 µm).

    • Variants: cocci (spherical), bacilli (rod-like), spirilla (coiled).

    • Most bacteria are harmless; some are virulent.

    • Examples:

      • Clostridium tetani (tetanus)

      • Streptococcal bacteria (strep throat)

22.1.2 Viruses

  • Characteristics:

    • Non-cellular; much smaller (one-hundredth of a micrometer).

    • Obligate intracellular parasites; must enter host cells to reproduce.

    • Cause cell death directly or via immune response.

    • Examples:

      • Common cold, Ebola, chickenpox

22.1.3 Fungi

  • Characteristics:

    • Eukaryotic cells with both membrane and cell wall.

    • Includes molds, yeasts.

    • Can cause superficial infections (e.g., ringworm) or systemic infections (e.g., histoplasmosis).

22.1.4 Protozoans

  • Characteristics:

    • Eukaryotic, lack cell walls.

    • Can be intracellular or extracellular parasites.

    • Example:

      • Malaria, trichomoniasis

22.1.5 Multicellular Parasites

  • Characteristics:

    • Macroscopic organisms that take nourishment from their host.

    • Example:

      • Tapeworms

22.1.6 Prions

  • Characteristics:

    • Infectious proteins, neither cells nor viruses.

    • Cause diseases in nervous tissue.

    • Example:

      • Variant Creutzfeldt-Jakob disease (mad cow disease).

22.2 Overview of the Immune System

  • Learning Objectives:

    1. List leukocytes of the immune system and their locations.

    2. Define cytokines and their similarities to hormones.

    3. List general categories of cytokines.

    4. Compare primary features of innate and adaptive immunity.

22.2a Immune Cells and Their Locations

  • Leukocytes (white blood cells):

    • Formed in red bone marrow.

    • Types:

      • Granulocytes (neutrophils, eosinophils, basophils)

      • Monocytes (become macrophages in tissues)

      • Lymphocytes (B-lymphocytes, T-lymphocytes, NK cells)

22.2b Cytokines

  • Cytokines: Small proteins that regulate immune activity.

    • Effects:

      • Signal cells, control immune cell development, regulate inflammatory responses, destroy cells.

    • Types:

      • Autocrine, paracrine, endocrine.

22.2c Comparison of Innate and Adaptive Immunity

  • Innate Immunity:

    • Present at birth, nonspecific, no prior exposure needed.

    • Features include physical (skin) and chemical barriers, cellular defenses.

  • Adaptive Immunity:

    • Acquired/specific; requires exposure to specific antigens.

    • Involves T- and B-lymphocytes; develops over several days.

22.3 Innate Immunity

  • Learning Objectives:

    1. Describe barriers to entry of harmful agents.

    2. Describe cells involved in nonspecific defenses (innate immunity).

    3. Explain functions of interferons.

    4. Define the complement system and how it is activated.

    5. Describe how the complement system participates in innate immunity.

22.3a First Line: Preventing Entry

  • Skin and Mucous Membranes:

    • Skin releases antimicrobial substances (e.g., dermicidin).

    • Mucous membranes produce mucus and antimicrobial substances (e.g., defensins, IgA).

22.3b Second Line: Nonspecific Internal Defenses

  • Phagocytic Cells:

    • Include neutrophils, macrophages, dendritic cells; engulf unwanted substances.

  • Chemicals / Cytokines:

    • Released by cells like basophils and mast cells; promote inflammation and attract immune cells.

  • NK Cells:

    • Destroy unhealthy cells by releasing perforin and granzymes, inducing apoptosis.

22.4 Adaptive Immunity: An Introduction

  • Learning Objectives:

    1. Describe features of antigens and antigenic determinants.

    2. Explain immunogenicity and its affecting attributes.

    3. Discuss how haptens stimulate immune responses.

    4. Describe receptors of T-lymphocytes and B-lymphocytes.

22.4a Antigens

  • Definition:

    • Substances that bind T-lymphocytes or antibodies; usually proteins or large polysaccharides.

  • Types:

    • Foreign antigens (non-self) vs. self-antigens.

    • Immune system usually recognizes self-antigens but can react in autoimmune disorders.

22.5 Formation and Selection of T-Lymphocytes

  • Learning Objectives:

    1. Explain T-lymphocyte maturation.

    2. Compare and contrast positive and negative selection in thymus.

22.5a Formation of T-Lymphocytes

  • Origin in red bone marrow; they migrate to thymus as pre-T-lymphocytes for maturation.

22.5b Selection and Differentiation of T-Lymphocytes

  • Thymic Selection:

    • Positive selection: survival of those that can bind MHC molecules.

    • Negative selection: removal of cells that bind self-antigens.

22.6 Activation and Clonal Selection of Lymphocytes

  • Learning Objectives:

    1. Describe activation mechanisms for T-lymphocytes and B-lymphocytes.

    2. Explain lymphocyte recirculation and its function.

22.6a Activation of T-Lymphocytes

  • Helper T-Lymphocytes:

    1. First signal: direct contact with MHC on APC.

    2. Second signal: cytokine release and cell proliferation.

  • Cytotoxic T-Lymphocytes:

    • Activated similarly, but target infected cells directly.

22.6b Activation of B-Lymphocytes

  • Activation Steps:

    1. Antigen binds to BCR and is presented to helper T-cell.

    2. Helper T-cell releases cytokines to stimulate B-cell.

22.7 Effector Response at Infection Site

  • Learning Objectives:

    1. Describe functions of T-lymphocytes and plasma cells during effector response.

22.7a Effector Response of T-Lymphocytes

  • Helper T-cells: regulate immune responses and activate other immune cells.

  • Cytotoxic T-cells: destroy unhealthy cells through apoptosis.

22.7b Effector Response of B-Lymphocytes

  • Plasma cells: produce and release antibodies, forming high antibody titers against specific antigens.

22.8 Immunoglobulins

  • Structure: Y-shaped proteins composed of heavy and light chains, with variable and constant regions.

  • Functions of Antibodies:

    • Neutralization, agglutination, precipitation, complement fixation, opsonization, activation of NK cells.

22.9 Immunologic Memory and Immunity

  • Learning Objectives:

    1. Define immunologic memory and explain its significance.

    2. Discuss differences between primary and secondary responses.

22.9a Immunologic Memory

  • Results in a long-lived army of memory lymphocytes; secondary responses are more rapid and effective.

22.9b Active and Passive Immunity

  • Active Immunity: Developed through exposure to pathogens; produces memory cells.

  • Passive Immunity: Transferred antibodies without memory cells being formed.