Lymphatic and Immune System Chapter 21 Part 2

Barrier Defenses

• The immune system consists of two mechanisms: innate (rapid, nonspecific) and adaptive (slower, highly specific).
• Barrier defenses continuously protect against pathogens before an infection occurs.
• Skin serves as the primary barrier, with dead, keratinized cells and antimicrobial properties of sweat lowering pH to prevent pathogen growth.
• Saliva contains lysozyme, which destroys bacterial cell walls, while the stomach's acidic environment kills many pathogens.
• Mucus in various tracts traps debris and pathogens, facilitating their removal.

Cells of the Innate Immune Response

• Phagocytes, like macrophages and neutrophils, engulf pathogens through phagocytosis, which is crucial in destroying invading organisms.
• Macrophages are versatile, irregularly shaped cells found in various tissues, acting as first responders to infections.
• Neutrophils are granulocytes that respond to infection via chemotaxis from the bloodstream, reinforcing the immune response at the infection site.
• Monocytes circulate in the blood and differentiate into macrophages or dendritic cells at sites of infection.

Natural Killer (NK) Cells

• NK cells induce apoptosis in cells infected with viruses or intracellular bacteria.
• They employ two main mechanisms: engaging fas ligand to trigger apoptosis and releasing perforins and granzymes to disrupt infected cell membranes.

Recognition of Pathogens

• Innate immune cells recognize pathogens through pattern recognition receptors (PRRs), identifying common bacterial components and triggering phagocytosis or apoptotic signaling.
• The innate immune system's response relies on a limited diversity of PRRs to manage a broad range of pathogens.

Soluble Mediators of the Innate Immune Response

• Cytokines and chemokines facilitate short-range cellular communication and attract immune cells from longer distances, respectively.
• Early induced proteins like interferons are critical for antiviral defenses, signaling neighboring cells to produce protective proteins.

Complement System

• The complement system consists of proteins in blood plasma, activated through alternative or classical pathways to enhance immune response.
• Functions include opsonization (tagging pathogens), chemotaxis (attracting phagocytes), and forming pores in pathogen membranes to induce cell lysis.

Inflammatory Response

• Inflammation, characterized by heat, redness, pain, and swelling, is a fundamental innate immune response to injury or infection.
• Tissue damage releases cellular contents that trigger inflammatory mediators (e.g., histamine, prostaglandins) to increase blood flow and vascular permeability.
• Recruitment of phagocytes, especially neutrophils, helps eradicate pathogens and clear debris.

Acute vs. Chronic Inflammation

• Acute inflammation is a short-term response leading to tissue repair, while chronic inflammation can cause significant damage and is linked to diseases like rheumatoid arthritis.### Adaptive Immune Response
• Unique ability to recognize and respond to a wide range of pathogens through specific antigens.
• Antigens are small chemical groups on pathogens recognized by receptors on B and T lymphocytes.
• Can develop around 100 trillion different receptors, enabling versatility in responding to pathogens.

Primary and Secondary Responses

• First exposure to a pathogen triggers a primary adaptive response, typically with severe symptoms.
• Secondary adaptive responses arise upon re-exposure to the same pathogen, faster and stronger, often preventing symptoms.
• Immunological memory provides lasting protection against previously encountered pathogens.

Self and Foreign Antigen Recognition

• Adaptive immune system can differentiate between self-antigens (body's own) and foreign antigens (pathogens).
• Mechanisms exist to prevent immune responses against self, but failures can lead to autoimmune diseases.

T Cell-Mediated Immune Responses

• T and B lymphocytes are key players in adaptive immunity, with T cells regulating many immune responses.
• T cells recognize antigens via two-chain protein receptors, primarily alpha-beta T cell receptors.
• Each T cell produces a single type of receptor specific to one antigen.

Antigens and Antigenic Determinants

• Antigens are complex structures with multiple antigenic determinants (epitopes), typically six or fewer amino acids or sugars.
• Protein antigens exhibit significant diversity in shape, essential for immune responses to viruses and parasites.

Antigen Processing and Presentation

• T cells recognize antigens only presented by specialized antigen-presenting cells (APCs).
• Antigens are internalized, processed, and displayed on Major Histocompatibility Complex (MHC) molecules on the cell surface.
• Two MHC classes:
  • MHC Class I mediates recognition of intracellular antigens.
  • MHC Class II handles extracellular antigens by professional APCs.

Professional Antigen-Presenting Cells

• Class II MHC is expressed on immune system cells:
  • Macrophages stimulate T cell cytokine release.
  • Dendritic cells transport antigens to lymph nodes for T cell activation.
  • B cells can also present antigens to T cells for antibody generation.

T Cell Development and Tolerance

• T cell tolerance eliminates self-reactive T cells, ensuring they do not attack body tissues.
• Thymocytes undergo positive selection to bind to self-MHC molecules and negative selection to avoid self-reactivity.

Mechanisms of T Cell Activation

• Activated T cells differentiate and proliferate through clonal expansion after recognizing foreign antigens with MHC.
• Clonal selection ensures only specific T cells against a pathogen expand, creating a polyclonal response due to multiple determinants.

Immunological Memory Development

• Memory T cells are generated during primary responses, allowing rapid secondary responses upon re-exposure, often overwhelming pathogens before disease symptoms arise.

T Cell Types and Functions

• T cells express either CD4 (helper T cells) or CD8 (cytotoxic T cells), aiding in immune responses via interaction with specific MHC molecules.
• Th cells release cytokines to bolster immune functions; Th1 regulates various immune cells while Th2 aids B cell differentiation and antibody production.

Cytotoxic T Cells and Viral Defense

• CD8+ cytotoxic T cells induce apoptosis in infected cells, crucial for controlling viral infections by preventing viral multiplication.

Regulatory T Cells

• CD4+ regulatory T cells (Treg) suppress other T cell functions, contributing to immune system moderation and tolerance, involved in preventing excessive immune responses.### Immune Response Regulation
• Uncontrolled clonal expansion during immune responses can lead to autoimmune diseases.
• T cells play a pivotal role in directly destroying pathogens and regulating various adaptive immune responses.
• Different types of T cells:
  • Tc (Cytotoxic T cells) target infected cells.
  • Th1 (Helper T cells) primarily activate macrophages.
  • Th2 (Helper T cells) primarily activate B cells.
  • Treg (Regulatory T cells) suppress immune responses.

T Cell Functions and Cytokines

• T cells utilize various surface markers:
  • Tc cells express CD8 and interact with Class I MHC molecules.
  • Th1 and Th2 cells express CD4 and engage with Class II MHC molecules.
• Cytokine production by T cells:
  • Tc cells produce perforins, granzymes, and fas ligand for cytotoxicity.
  • Th1 cells produce interferon-γ and TGF-β.
  • Th2 cells secrete IL-4, IL-6, IL-10, among others.
  • Treg cells utilize TGF-β and IL-10 to suppress other immune cells.

B Cell Maturation and Activation

• B cells mature in the bone marrow, generating a vast number of unique clones, akin to T cell receptor diversity.
• Central tolerance eliminates or inactivates B cells that strongly bind to self-antigens, preventing autoimmunity through:
  • Clonal deletion: Apoptosis of immature B cells binding to self-antigens.
  • Clonal anergy: Functional inactivation without deletion for B cells exposed to soluble antigens.
• Peripheral tolerance occurs when matured B cells do not encounter self-antigens, maintaining immune regulation.

Antibody Function and Structure

• Antibodies, or immunoglobulins, are secreted forms of B cell receptors, identifiable by their ability to agglutinate pathogens.
• Antibody classes in humans include IgM, IgD, IgG, IgA, and IgE, each with distinct functions.
• B cells can recognize native antigens independently, without the need for MHC molecules or antigen-presenting cells.

Plasma Cells and Memory B Cells

• Activated B cells differentiate into plasma cells that produce specific antibodies and often migrate back to the bone marrow.
• Plasma cells are terminally differentiated, expending energy on antibody production rather than self-maintenance.
• Memory B cells arise from the clonal expansion of activated B cells, enabling a rapid and robust response during secondary encounters with the same antigen.

Antibody Structure

• Antibodies consist of two heavy chains and two light chains forming a four-chain structure, essential for their function.
• The Fc region, formed by heavy chains, facilitates binding to immune effector cells and enhances pathogen specificity.
• Antibodies possess two identical antigen-binding sites crucial for the immune response against pathogens.

This quiz covers the barrier defenses and the innate immune response as detailed in Biology Chapter 21.2. You will learn about how the body protects itself from pathogens and the roles of various soluble factors in the immune process. Additionally, the inflammation steps and the importance of early immune responses will be explored.