Immune System Overview

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Questions and Answers

What is the primary function of innate immunity?

To recognize nonmicrobial substances

To mediate responses via antibodies

To adapt to microbes after exposure

To block entry of environmental microbes (correct)

Which type of immunity is primarily responsible for defense against intracellular microbes?

Humoral immunity

Cell-mediated immunity (correct)

Innate immunity

Natural killer cell activity

What is the approximate CD4:CD8 ratio in T-lymphocytes?

1:1

3:1

2:1 (correct)

4:1

Which receptor is primarily involved in signaling through T-lymphocytes?

T cell receptor (TCR) Signup and view all the answers

Which interleukin is not produced by T-helper-1 (TH1) cells?

IL-5 Signup and view all the answers

What component is linked to the T cell receptor for signal transduction?

CD3 complex Signup and view all the answers

Which type of lymphocyte is primarily responsible for humoral immunity?

B-lymphocytes Signup and view all the answers

What distinguishes adaptive immunity from innate immunity?

Ability to remember past infections Signup and view all the answers

What characterizes the immediate phase of a Type I hypersensitivity reaction?

Vasodilatation and exudation due to vasoactive amines Signup and view all the answers

Which of the following is an example of a Type II hypersensitivity reaction?

Transfusion reaction Signup and view all the answers

What is the primary mechanism by which Type III hypersensitivity reactions cause tissue damage?

Deposition of immune complexes in tissues Signup and view all the answers

Which T cell type is primarily involved in the delayed hypersensitivity reactions of Type IV hypersensitivity?

CD4+ T cells Signup and view all the answers

Which of the following is NOT a mechanism of Type II hypersensitivity?

Back-channel communication Signup and view all the answers

Which of the following diseases is associated with Type III hypersensitivity?

Serum sickness Signup and view all the answers

In Type IV hypersensitivity, which cells are primarily responsible for direct cell cytotoxicity?

CD8+ T cells Signup and view all the answers

What defines the late phase of a Type I hypersensitivity reaction?

Inflammation due to leukocyte influx Signup and view all the answers

What is the main function of CD8+ T cells?

Kill other cells Signup and view all the answers

Which two signals are essential for complete T cell activation?

MHC-antigen complex engagement and CD28 interaction Signup and view all the answers

What percentage of circulating lymphocytes do B-lymphocytes constitute?

10-20% Signup and view all the answers

What is the role of CD40 on B cells?

Interact with CD154 on activated T-lymphocytes Signup and view all the answers

What distinguishes follicular dendritic cells from other dendritic cells?

Antibodies on their cell surfaces Signup and view all the answers

What is a primary function of macrophages in the immune response?

Present antigens to T cells Signup and view all the answers

How do B-cells change after antigen stimulation?

They transform into plasma cells Signup and view all the answers

Which type of immunoglobulin is found in traces in the serum?

IgE Signup and view all the answers

What percentage of peripheral blood lymphocytes do natural killer cells constitute?

10-15% Signup and view all the answers

Which receptors are used to identify natural killer cells?

CD16 and CD56 Signup and view all the answers

What type of antigens do class I MHC molecules present?

Endogenous viral peptides Signup and view all the answers

Which of the following correctly describes the function of CD8 molecules?

They serve as coreceptors for MHC class I-restricted T cells. Signup and view all the answers

Which class of MHC molecules presents exogenous antigens?

Class II MHC Signup and view all the answers

Where are the genes encoding MHC molecules located?

Chromosome 6 Signup and view all the answers

What is the main function of natural killer (NK) cells?

To kill tumor cells and virally-infected cells Signup and view all the answers

Which statement about class III MHC gene products is true?

They encode components of the complement system. Signup and view all the answers

Study Notes

Introduction

The immune system defends against infection through two mechanisms: innate and adaptive immunity

Innate Immunity

Present at birth and acts immediately
Recognizes and targets common microbial features
Components include:
- Epithelial barriers: Prevent entry of microbes
- Phagocytic cells: Neutrophils and macrophages engulf and destroy microbes
- Natural Killer (NK) cells: Destroy infected or cancerous cells
- Complement system: A group of plasma proteins that help in the destruction of microbes

Adaptive Immunity

Develops after exposure to specific pathogens
Highly specific to each pathogen
Consists of two types:
- Cell-mediated immunity (Cellular): Targets intracellular microbes. Mediated by T lymphocytes (T cells)
- Humoral immunity: Targets extracellular microbes and their toxins. Mediated by B lymphocytes (B cells) and their secreted antibodies

Immune System Cells

T lymphocytes (T cells):
- 60-70% of circulating lymphocytes
- Found in lymph nodes (paracortical area) and spleen (periarteriolar sheath)
- Each T cell carries a unique T-cell receptor (TCR) recognizing specific antigens
  - The TCR is made of α and β polypeptide chains
    - Each chain has a variable (antigen-binding) region and a constant region
  - The TCR is linked to the CD3 complex involved in signaling
- CD4 and CD8: coreceptors on T cells, expressed in different subsets
  - CD4+: bind to class II MHC molecules on antigen-presenting cells
    - About 60% of T cells
  - **CD8+: ** bind to class I MHC molecules
    - About 30% of T cells
  - CD4:CD8 ratio: approximately 2:1
- T-helper (TH) cells:
  - TH1 cells: produce IL-2 and IFN-γ (involved in macrophage activation and delayed hypersensitivity)
  - TH2 cells: produce IL-4, IL-5, and IL-13 (involved in B-cell antibody production and eosinophil activation)
- CD8+ cytotoxic T cells: kill infected or cancerous cells
B lymphocytes (B cells):
- 10-20% of circulating lymphocytes
- Found in lymph nodes (superficial cortex) and spleen (white pulp)
- Transform into plasma cells upon activation
- Plasma cells secrete antibodies (immunoglobulins):
  - IgG, IgM, IgA: mainly in plasma
  - IgE: trace amounts in serum
  - IgD: cell-bound to B cells
- B-cell receptor (BCR): monomeric IgM on the surface of B cells, recognizes antigens
- CD 19, CD20 and CD21: expressed on B cells
  - CD21 binds to Epstein-Barr virus (EBV)
  - CD40 interacts with CD154 on activated T cells
Macrophages:
- Play multiple roles in the immune response:
  - Antigen presentation: Present antigens to T cells via class II MHC molecules
  - Cytokine production: Influence T, B, endothelial, and fibroblast cells
  - Immune effector cells: Produce toxic metabolites and enzymes to kill tumor cells
  - Delayed hypersensitivity: Important effector cell in type IV hypersensitivity
Dendritic cells (DCs):
- Have dendritic cytoplasmic processes enabling efficient antigen presentation
- Found in lymphoid tissue and epidermis
- Langerhans cells: DCs found in the epidermis
  - Efficient antigen presentation with poor phagocytic activity.
- Follicular dendritic cells: Found in germinal centers
  - Contain antibodies on their surface, trapping antigens via Fc receptors
Natural Killer (NK) cells:
- 10-15% of peripheral blood lymphocytes
- larger than lymphocytes with large granules (large granular lymphocytes)
- Part of the innate immune system, kill tumor cells and virus-infected cells without prior sensitization
- Characterized by expression of:
  - CD16: Fc receptor for IgG-coated cells, mediating antibody-dependent cell-mediated cytotoxicity (ADCC)
  - CD56: NK cell marker
- Express receptors:
  - Activators: recognize unknown molecules on target cells
  - Inhibitors: recognize class I MHC molecules
  - All nucleated normal cells express class I MHC, preventing healthy cells from being killed
- Secrete IFN-γ

Major Histocompatibility Complex (MHC)

Presents peptide fragments of foreign proteins to T cells
Genes encoding for MHC molecules are found on chromosome 6, called Human Leukocyte Antigen (HLA)
Three classes:
- Class I:
  - Expressed on all nucleated cells and platelets
  - Encoded by HLA-A, HLA-B, and HLA-C
  - Bind peptides from proteins synthesized inside the cell (e.g., viral antigens)
  - Recognized by CD8+ cytotoxic T cells: class I MHC-restricted
- Class II:
  - Encoded by HLA-DP, HLA-DQ, and HLA-DR
  - Bind peptides from proteins internalized and processed outside the cell (e.g., extracellular microbes)
  - Recognized by CD4+ helper T cells: class II MHC-restricted.
- Class III:
  - Encode components of the complement system

Hypersensitivity Reactions

Immune reactions that cause disease
Four types:
- Type I (Immediate):
  - Mediated by IgE antibodies binding to mast cells
  - Rapid onset (minutes) after exposure to allergen
  - Two phases:
    - Immediate phase: Vasodilation and exudation due to the release of vasoactive amines
    - Late phase: Inflammatory cell infiltration (e.g., in asthma and allergic rhinitis)
- Type II (Antibody-dependent):
  - Mediated by antibodies directed towards antigens on cell surfaces or extracellular matrix
  - Antigens can be intrinsic (self) or extrinsic (foreign)
  - Causes cell destruction through three mechanisms:
    - Opsonization and phagocytosis: Antibody coating targets cells for engulfment by phagocytes
    - Antibody-dependent cellular cytotoxicity (ADCC): NK cells kill antibody-coated target cells
    - Antibody-mediated cellular dysfunction: Antibodies interfere with cell function (e.g., myasthenia gravis, Graves' disease)
  - Examples: Transfusion reactions, autoimmune hemolytic anemia, drug reactions, glomerulonephritis, organ rejection
- Type III (Immune Complex-mediated):
  - Antigen-antibody complexes deposit in tissues, triggering inflammation
  - Circulating immune complexes can deposit in various organs, particularly blood vessels
  - Examples: Systemic lupus erythematosus, polyarteritis nodosa, poststreptococcal glomerulonephritis, serum sickness
- Type IV (Cell-mediated):
  - Mediated by antigen-activated T lymphocytes
  - Delayed onset (hours to days)
  - Two subsets:
    - Delayed type hypersensitivity (DTH): Mediated by CD4+ T cells
      - Important in defense against intracellular pathogens (mycobacteria, fungi, parasites)
      - Also involved in transplant rejection and tumor immunity
    - Direct cytotoxicity: Mediated by CD8+ T cells
      - Mainly involved in graft rejection

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Description

Explore the key concepts of the immune system, including innate and adaptive immunity. This quiz covers the mechanisms and components involved in defending against infections, such as epithelial barriers, phagocytic cells, and T and B lymphocytes. Test your knowledge on how these systems work together to protect the body.