Overview
This lecture explores the body's last line of immune defense: the cell-mediated (cellular) immune response, focusing on T cells, their activation, and how imbalances can lead to autoimmune diseases.
Barriers and Initial Defenses
- Skin and mucous membranes act as physical barriers to infection.
- The humoral immune response uses antibodies to neutralize pathogens in body fluids.
- When these defenses fail and pathogens enter cells, the cellular immune response is activated.
Cellular Immune Response
- The cell-mediated response targets infected or abnormal body cells with T lymphocytes (T cells).
- T cells can cause inflammation, activate macrophages, and regulate the immune response.
Antigen Presentation & MHC Complexes
- Major histocompatibility complexes (MHCs) display antigen fragments on cell surfaces.
- All nucleated cells have class I MHCs presenting endogenous (internal) protein fragments.
- Professional antigen presenting cells (macrophages, dendritic cells, B cells) use class II MHCs to display exogenous (external) antigen fragments.
T Cell Types and Actions
- T cells mature in the thymus (“T” for thymus).
- Helper T cells coordinate immune responses by releasing cytokines but do not kill infected cells.
- Cytotoxic T cells directly kill infected or abnormal cells by inducing apoptosis.
- Regulatory T cells help shut down immune responses to prevent damage to healthy tissue.
Activation and Checks and Balances
- Helper T cells recognize specific MHC-antigen combinations and activate, proliferating and releasing cytokines.
- Cytokines stimulate B cells and cytotoxic T cells, amplifying the immune response.
- B cells require confirmation from Helper T cells before full activation to avoid attacking self-proteins.
Immunodeficiency and Autoimmunity
- Immunodeficiencies, like AIDS, are dangerous because they target Helper T cells, crippling adaptive immunity.
- Autoimmune diseases result from immune system errors, attacking the body's own tissues (e.g., multiple sclerosis, Type 1 diabetes).
- Regulatory T cells inhibit immune responses after threats are neutralized, preventing autoimmunity.
Key Terms & Definitions
- T lymphocytes (T cells) — White blood cells that mediate cellular immunity.
- MHC (Major Histocompatibility Complex) — Proteins displaying antigen fragments on cell surfaces.
- Antigen presenting cell — Cell that displays antigen fragments via MHC for T cell recognition.
- Helper T cell — Coordinates immune response, activates other immune cells via cytokines.
- Cytotoxic T cell — Kills infected or abnormal cells by inducing programmed cell death.
- Cytokines — Chemical messengers that stimulate and regulate immune cells.
- Regulatory T cell — Suppresses immune responses to prevent attack on self-tissues.
- Autoimmune disease — Condition where the immune system attacks the body’s own cells.
Action Items / Next Steps
- Review the roles and activation processes of Helper, Cytotoxic, and Regulatory T cells.
- Understand how MHC complexes function in antigen presentation.
- Prepare for questions on autoimmune diseases and immunodeficiency mechanisms.