The Immune System: Innate and Adaptive Immunity, CAR T Cells, Vaccines, and Autoimmunity

Study Guide - Smart Notes

Tailored notes based on your materials, expanded with key definitions, examples, and context.

Immune System Overview

The immune system is a complex network of cells, tissues, and molecules that defends the body against pathogens. It is divided into two main branches: innate immunity and adaptive immunity, each with distinct mechanisms and cell types for recognizing and eliminating foreign invaders.

Innate vs. Adaptive Immunity

Innate and adaptive immunity are the two primary defense strategies of the immune system, differing in their specificity, memory, and cellular components.

Innate Immunity: The first line of defense, providing rapid but non-specific responses to pathogens.
Adaptive Immunity: A slower, highly specific response that develops memory for faster responses upon re-exposure to the same pathogen.

Key Differences

Feature	Innate Immunity	Adaptive Immunity
Speed of Response	Immediate (minutes to hours)	Delayed (days)
Specificity	Non-specific	Highly specific (antigen-dependent)
Memory	None	Long-lasting memory
Main Cell Types	Phagocytic cells (macrophages, neutrophils), dendritic cells, natural killer cells	B cells, T cells (helper and cytotoxic)

Barrier Defenses

Physical barriers: Skin, mucous membranes
Chemical barriers: Enzymes in saliva, stomach acid

Phagocytic Cells

Macrophages: Engulf and digest pathogens; present antigens to T cells.
Neutrophils: Rapid responders that ingest and destroy microbes.
Dendritic Cells: Capture antigens and activate adaptive immunity by presenting antigens to T cells.

Toll-like Receptors (TLRs)

Pattern recognition receptors on innate immune cells that detect conserved microbial molecules.

Adaptive Immunity: Cell-Mediated and Humoral Responses

Adaptive immunity is divided into cell-mediated and humoral responses, each involving specialized lymphocytes.

Cell-mediated immune response: Involves T cells that directly kill infected cells or help other immune cells.
Humoral immune response: Involves B cells that produce antibodies to neutralize pathogens.

B Cells and Antibodies

B cell receptor (BCR): Membrane-bound antibody that recognizes specific antigens.
Antibody: Secreted protein that binds to antigens, marking them for destruction.
Plasma cell: Differentiated B cell that secretes large amounts of antibody.
Memory B cell: Long-lived cell that responds rapidly upon re-exposure to the same antigen.

T Cells

T cell receptor (TCR): Recognizes antigen fragments presented by MHC molecules on other cells.
Helper T cell (CD4+): Recognizes antigens presented by MHC class II molecules on antigen-presenting cells; secretes cytokines to activate B cells and cytotoxic T cells.
Cytotoxic T cell (CD8+): Recognizes antigens presented by MHC class I molecules on infected or cancerous cells; induces apoptosis in these cells.
Memory helper/cytotoxic T cells: Persist after infection to provide rapid response upon re-infection.

Antigen Presentation

Antigen-presenting cells (APCs): Cells such as dendritic cells, macrophages, and B cells that display antigen fragments on MHC molecules to T cells.
MHC class I: Present on all nucleated cells; present intracellular antigens to cytotoxic T cells.
MHC class II: Present on APCs; present extracellular antigens to helper T cells.

CAR T Cells and Cancer Immunotherapy

Chimeric Antigen Receptor (CAR) T cells are engineered T cells used in cancer therapy. They differ from normal cytotoxic T cells in how they recognize and kill cancer cells.

CAR T cells: T cells genetically modified to express chimeric antigen receptors that recognize specific proteins on cancer cells, independent of MHC presentation.
Normal cytotoxic T cells: Recognize cancer antigens presented by MHC class I molecules via their TCRs.

Key Concepts:

Antigen: Any molecule that can be recognized by an antibody or TCR.
T cell receptor (TCR): Recognizes antigen-MHC complexes.
Antigen-presenting cells (APCs): Present antigens to T cells using MHC molecules.
Cytotoxic T cells: Kill infected or cancerous cells displaying antigens on MHC class I.

Comparison Table: CAR T Cells vs. Normal Cytotoxic T Cells

Feature	CAR T Cells	Normal Cytotoxic T Cells
Antigen Recognition	Directly via engineered receptor (not MHC-dependent)	Via TCR recognizing antigen on MHC class I
Source	Genetically engineered in the lab	Develop naturally in the body
Use in Therapy	Cancer immunotherapy	Normal immune surveillance

Vaccines and Immune Memory

Vaccines stimulate the adaptive immune system to generate a protective response without causing disease. They rely on the concepts of antigen, antibody, B cells, helper T cells, plasma cells, clonal selection, and memory B cells.

Antigen: Component of the vaccine that mimics a pathogen.
B cells: Recognize antigen and differentiate into plasma cells and memory B cells.
Plasma cells: Produce antibodies specific to the antigen.
Helper T cells: Activate B cells and enhance antibody production.
Clonal selection: Process by which only B cells with receptors specific to the antigen proliferate.
Memory B cells: Provide long-term immunity by responding rapidly to future exposures.

How Vaccines Work:

Vaccine introduces antigen into the body.
B cells recognize antigen and, with help from helper T cells, undergo clonal selection and differentiation.
Plasma cells produce antibodies; memory B cells are generated for long-term protection.

Self-Tolerance and Autoimmune Disease

The immune system typically avoids attacking the body's own tissues through mechanisms of self-tolerance. Failure of these mechanisms can lead to autoimmune diseases.

Self-tolerance: The ability of the immune system to avoid attacking self-antigens.
Mechanisms of self-tolerance:
- Elimination of self-reactive lymphocytes during development (central tolerance).
- Suppression of self-reactive cells by regulatory T cells (peripheral tolerance).
Autoimmune disease: Condition in which the immune system attacks the body's own tissues.
Mechanisms for autoimmunity:
- Failure to eliminate self-reactive lymphocytes.
- Cross-reactivity, where immune responses to foreign antigens also target similar self-antigens.

Key Terms and Definitions

Pathogen: Disease-causing organism (e.g., bacteria, viruses, fungi, parasites).
Phagocytosis: Process by which cells engulf and digest pathogens.
Cytokine: Signaling molecule that modulates immune responses.
Intracellular pathogen: Pathogen that lives inside host cells (e.g., viruses).
Extracellular pathogen: Pathogen that lives outside host cells (e.g., most bacteria).
Vaccine: Preparation containing antigens that stimulate an immune response and memory formation.
Cross-reactivity: When an immune response to one antigen also reacts with a structurally similar self-antigen.

Additional info: The above content expands on the provided outline by including definitions, mechanisms, and examples for each key concept, as would be expected in a comprehensive study guide for the immune system.