Chapter 15: Immune Reactions to Antigens and the Activities of T cells

Chapter 15: Immune Reactions to Antigens and the Activities of T Cells

15.3 Immune Reactions to Antigens

Cooperation in Immune Reactions to Antigens
  • The basis for most immune responses is the encounter between antigens and white blood cells, primarily:
    • Neutrophils
    • T cells
    • B cells
  • Lymph nodes and spleen play a crucial role by concentrating the antigens and circulating them to facilitate contact with antigen-presenting cells (APCs) and lymphocytes.
The Role of Antigen Processing and Presentation
  • T-cell dependent antigens require processing by APCs before they can interact with T cells.
  • The three cell types that can serve as APCs are:
    • Macrophages
    • Dendritic Cells (most common)
    • B Cells
Steps of Antigen Processing and Presentation
  1. Processing: APCs process the antigen by breaking it down into smaller fragments.
  2. Presentation: The processed antigen is moved to the APC surface and bound to an MHC class 2 receptor.
  3. Binding: T cells can then bind to the antigen:MHC class 2 complex.
Antigen Presentation for T Cell Activation
  • APCs first engulf microbes and degrade them into smaller peptides.
  • The antigen peptides are then loaded onto MHC class 2 receptors that are displayed on the APC surface.
Interaction of T Helper Cells with APCs
  • The T helper cell recognizes receptors on the APC through the following steps:
    1. T cell receptor binds to the MHC class II:Antigen complex.
    2. CD4 coreceptor binds to the MHC.
    3. CD28 binds to CD80 on APC.
  • The binding of these cells, along with the presence of cytokines, results in T cell activation.
Importance of Signals for T Cell Activation
  • All signals mentioned above are critical for effective T cell activation.
  • Upon activation, CD4+ T cells may differentiate into various cell types:
    • Memory T cells
    • T helper effector cells
    • Th17 cells
    • T regulatory cells
CD4+ T Cells (T Helper cells)
  • CD4+ T cells, also known as T Helper (TH) cells, are characterized by:
    • Expression of CD4 receptors and activation by antigen/MHC II.
    • Prevalence in blood and lymphoid organs.
    • Regulation of immune reactions to antigens, supporting both other T and B cells.
  • Functions of CD4+ T cells:
    • Activate macrophages and enhance phagocytosis.
    • Differentiation into TH1 or TH2 cells depends on the cytokines released by APCs:
    • TH1: Activates more T cells.
    • TH2: Encourages B cell differentiation and an antibody response.
CD8+ T Cells (Cytotoxic T cells)
  • CD8+ T cells are also known as Cytotoxic T (Tc) cells:
    • Characterized by the expression of CD8 receptors and activation by antigen/MHC I.
    • They are powerful cells that destroy virus-infected cells, cancer cells, and foreign cells from other organisms.
    • Mechanisms of destruction:
    • Perforins: Proteins that punch holes in target cell membranes.
    • Granzymes: Enzymes that digest proteins in target cells.
Types of T Cells
  • Table 15.2 Characteristics of Subsets of T Cells
    • T Helper Cell 1 (TH1):
    • Primary Receptor: CD4
    • Function: Activates both CD4 and CD8 cells, secretes IL-2, tumor necrosis factor, and interferon gamma; responsible for delayed hypersensitivity; interacts with MHC-II receptors.
    • T Helper Cell 2 (TH2):
    • Primary Receptor: CD4
    • Function: Drives B-cell proliferation; secretes IL-4, IL-5, IL-6, IL-9, IL-10, and IL-13; can dampen TH1 activity.
    • T Helper 17 (TH17):
    • Primary Receptor: CD4
    • Function: Promotes inflammation; secretes IL-17.
    • T Regulatory Cell (Treg):
    • Primary Receptors: CD4, CD25
    • Function: Involved in the development of immune tolerance; suppresses harmful immune responses, inflammation, and autoimmunity.
    • T Cytotoxic Cell (TC):
    • Primary Receptor: CD8
    • Function: Destroys target foreign cells via lysis; crucial in cancer cell destruction, virus-infected cell eradication, and graft rejection; requires MHC I for functionality and may have some regulatory functions.