Chapter 21 C - BLG 102 - The Lymphatic System and Lymphoid Organs and Tissues

Human Anatomy and Physiology: The Immune System

Chapter Overview

  • Focus: Innate and Adaptive Body Defenses

  • Lecture Slides prepared by: Karen Dunbar Kareiva, Ivy Tech Community College

  • Copyright Information: Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved

21.6 Cellular Immune Response

  • T cells are crucial for defending against intracellular antigens.

    • Intracellular Antigens: Include cells infected with viruses or bacteria, cancerous or abnormal cells, and foreign (transplanted) cells.

    • Function: Some T cells directly kill infected or abnormal cells while others release chemicals to regulate the immune response.

T Cell Classifications and Functions

  • Complexity: T cells are more complex than B cells in both classification and function.

  • Two Major Populations of T Cells:

    • CD4 Cells:

    • Typically become Helper T Cells (TH).

    • Functions:

      • Activate B cells.

      • Activate other T cells.

      • Activate macrophages.

      • Direct overall adaptive immune response.

    • Some develop into Regulatory T Cells (moderate immune response).

    • Can differentiate into Memory T Cells that respond upon re-exposure to the same antigen.

    • CD8 Cells:

    • Differentiate into Cytotoxic T Cells (TC) that destroy infected or abnormal cells.

    • Also become Memory T Cells for future immune response.

Activation of T Cells

  • Types of T Cells:

    • Helper, Cytotoxic, and Regulatory T Cells are activated T cells.

    • Naive T Cells referred to simply as CD4 or CD8 cells until activated.

  • Role of Lymphoid Tissues and Organs:

    • Thymus involved in adaptive defenses and cellular immunity.

    • Immature lymphocytes undergo maturation, depending upon their T cell receptor type (CD4 or CD8).

MHC Proteins and Antigen Presentation

  • T cells only respond to processed fragments of antigens displayed by Major Histocompatibility Complex (MHC) proteins.

  • Antigen Presentation: Essential for the activation of naive T cells and normal functioning of effector T cells.

Types of MHC Proteins

  1. Class I MHC Proteins:

    • Displayed by all cells apart from Red Blood Cells (RBCs).

    • Bind short fragments (8–9 amino acids) of endogenous antigens (proteins synthesized inside the cell).

      • Endogenous Antigens can be:

      • Self Antigens: Normal proteins of the healthy cell.

      • Nonself Antigens: Abnormal proteins related to infected or cancerous cells.

    • Key Role: Enable CD8 cell activation; act as antigen holders, providing a “self” part recognized by T cells.

  2. Class II MHC Proteins:

    • Displayed by Antigen-Presenting Cells (APCs) including dendritic cells, macrophages, and B cells.

    • Bind longer fragments (14–17 amino acids) of exogenous antigens which are engulfed and processed within phagolysosomes.

    • Recognized by helper T cells, signaling the requirement for assistance.

Activation and Differentiation of T Cells

  • Activation Process: T cells can only be activated through antigen presentation.

    • Two-Step Activation:

    1. Antigen Binding: Binding of T cell receptors (TCRs) to antigen-MHC complex on the APC surface.

      • TCR must recognize both MHC and foreign antigen.

      • Initiates multiple intracellular signaling pathways starting T cell activation.

    2. Co-stimulation: Requires binding to one or more co-stimulatory signals on the APC surface.

    • Both steps must occur on the surface of the same APC for effective clonal selection of T cells.

  • Proliferation and Differentiation:

    • Activated T cells enlarge and proliferate in response to cytokines.

    • Differentiate and perform unique functions according to their T cell subtype (Helper, Cytotoxic).

    • Primary T cell response peaks within one week of initial exposure.

Roles of Specific Effector T Cells

  • Helper T Cells (TH):

    • Central role in adaptive immune responses.

    • Functions include:

    • Activation of B cells and other T cells.

    • Induction of T and B cell proliferation.

    • Secretion of cytokines that recruit additional immune cells.

    • Absence of TH cells results in no effective immune response.

  • Activation of B Cells:

    • Helper T cells interact with B cells displaying antigen fragments bound to MHC II receptors.

    • TH cells stimulate B cells to rapidly divide and start antibody production.

Central Role of Helper T Cells

  • Illustrative of how helper T cells mobilize both humoral and cellular immunity:

    1. Interaction with B Cells: Binding to self-nonself complexes displayed on B cells via MHC II.

    2. Activation Process: Helper T cells release interleukins as co-stimulatory signals to complete B cell activation.

  • Activation of CD8 Cells: Requires TH cells to help transform naive CD8 cells into cytotoxic T cells.

    • TH cells cause dendritic cells to express necessary co-stimulatory molecules for CD8 cell activation.

Summary of Important Figures and Tables

  • Figure 21.16: Major types of T cells and their differentiation.

  • Table 21.6: Role of MHC Proteins in Cellular Immunity.

  • Table 21.8: Cells and Molecules of the Adaptive Immune Response.

Midterm Information

  • Exam Scheduled: Monday, Feb. 9th from 3:10-5:10 PM.

  • Coverage: Lecture topics will include:

    • Cardiovascular system: heart (parts 1 and 2)

    • Cardiovascular system: blood vessels

    • Lymphatic and immune system

  • Format: Combination of multiple-choice, true/false, and diagram labeling questions.

  • Requirements: Bring student ID, pen, and pencil. No aids permitted in the exam.