pt 2

Overview of Viruses and Cytotoxic T Cells

  • Viruses invade host cells, utilizing cellular mechanisms to replicate and produce viral proteins.

    • The viral proteins are presented on the cell surface, alerting the immune system to infection.

    • The replication of viruses can lead to the destruction (lysis) of host cells, promoting the spread of infection.

    • The virus functions like a "come and kill me" signal, indicating the need for immune response activation to prevent further viral spread.

Cytotoxic T Cells and Activation

  • Cytotoxic T (T c) cells play a crucial role in identifying and destroying infected cells.

    • Inactive T c cells recognize specific shapes known as antigens, which are displayed by body cells.

    • Each T cell expresses a unique T cell receptor (TCR) that has specificity for a single antigen shape.

  • T cell activation process:

    1. Recognition of Antigen:

    • Activated by locking onto the antigen presented on the MHC Class I molecule of an infected cell.

    1. Co-stimulation:

    • Requires interaction with CD8 (cluster differentiation protein 8) to confirm activation.

    1. Upon successful binding and co-stimulation, the T cell proliferates in a process known as clonal expansion.

Clonal Expansion and Memory Cells

  • Clonal expansion results in the replication of T c cells that recognize the same antigen, significantly increasing their numbers (e.g., from 1000 to 3000).

  • Some T c cells become memory T cells, which remain inactive but are primed for future encounters with the same pathogen.

Mechanism of Action of Cytotoxic T Cells

  • Upon activation, T c cells execute infected cells via:

    1. Cytokine Release:

    • Stimulates apoptosis (programmed cell death) in the infected cells.

    1. Perforins:

    • Proteins that perforate the infected cell membrane, leading to a loss of cellular integrity and eventual cell death.

    1. Lymphotoxins:

    • Toxic proteins that disrupt metabolic functions within the infected cell, further promoting cell death.

Role of Helper T Cells

  • Helper T (T h) cells are essential for coordinating the immune response.

    • T h cells recognize antigens presented by MHC Class II molecules on antigen presenting cells (APCs).

    • T h cells also express CD4, distinctive from the CD8 of T c cells.

    • Activation of T h cells leads to:

    • Clonal expansion and differentiation into various effector cells, including cytokine-secreting cells that help B cells.

Activation of B Cells

  • B cells are produced and mature in the bone marrow.

  • Each B cell expresses a unique antibody on its surface, which serves as a receptor for antigens.

  • B cell activation involves:

    1. Antigen Recognition:

    • B cells bind to specific antigens via their antibodies and endocytose them, processing the antigen and presenting it on MHC Class II molecules.

    1. Helper T Cell Activation:

    • B cells require help from activated T h cells, which provide signals through cytokine release and direct interaction to promote B cell proliferation and differentiation.

  • Upon activation, B cells undergo clonal expansion and can develop into:

    1. Plasma Cells:

    • Specialized to produce and secrete antibodies into circulation, effectively neutralizing pathogens.

    1. Memory B Cells:

    • Remain in a dormant state for quicker responses upon re-exposure to the same antigen.

Antibody Structure and Function

  • Antibodies (also known as immunoglobulins) consist of:

    • Two heavy chains and two light chains, forming a Y-shaped structure.

    • Antigens are bound at the tips, determining specificity through variable regions.

  • Antibodies facilitate immune response by:

    1. Marking pathogens for destruction by phagocytes.

    2. Binding to antigens to inhibit their function.

    3. Activating complement pathways to lyse pathogens directly.

Memory Cells and Immune Response Regulation

  • Memory T and B cells are crucial for rapid responses during subsequent infections by the same pathogen.

  • Regulatory T (T reg) cells function to prevent overactivation of the immune system, secrete cytokines that help slow down immune responses after an infection is cleared.

Conclusions on Immune Mechanisms

  • Understanding the detailed interactions between various immune cells (e.g., T c, T h, B cells) enhances comprehension of effective immune responses and the management of infections.

  • The immune system's complexity underscores the necessity of rigorous regulatory mechanisms to maintain balance and prevent pathological conditions like autoimmune diseases.