Immune System Overview

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Questions and Answers

What is a primary function of the innate immune system?

To develop immunological memory.

To facilitate antigen presentation.

To produce antibodies for longer-lasting immunity.

To stimulate a quick inflammatory response. (correct)

Which cells are primarily responsible for producing type I interferons?

T lymphocytes

Macrophages

NK cells (correct)

B lymphocytes

Which type of immunity is mediated by B lymphocytes?

Innate immunity

Cell-mediated immunity

Adaptive cellular immunity

Humoral immunity (correct)

What distinguishes adaptive immunity from innate immunity?

Adaptive immunity can generate memory responses. Signup and view all the answers

Which of the following organs is classified as a primary lymphoid organ?

Thymus Signup and view all the answers

What role do antigen-presenting cells play in the adaptive immune response?

They present antigens to lymphocytes for recognition. Signup and view all the answers

What type of immunity primarily protects against intracellular microbes?

Cell-mediated immunity Signup and view all the answers

Which lymphoid organ is responsible for B cell development?

Bone marrow Signup and view all the answers

What is the primary role of the immune system in the human body?

To protect against infectious pathogens and foreign substances Signup and view all the answers

Which of the following is NOT a component of innate immunity?

T lymphocytes Signup and view all the answers

What are antibodies primarily produced in response to?

Antigenic stimulation Signup and view all the answers

Which of these statements best defines a hapten?

A small molecule that can only elicit an immune response when combined with a larger molecule Signup and view all the answers

Which of the following classes of immunoglobulins is primarily involved in allergic reactions?

IgE Signup and view all the answers

What is a major function of natural killer cells?

To destroy virus-infected cells and tumor cells Signup and view all the answers

What is the first line of defense in the immune system?

Innate immunity Signup and view all the answers

Which of the following describes the difference between innate and adaptive immunity?

Adaptive immunity involves memory, while innate immunity does not Signup and view all the answers

What is the primary function of Natural Killer (NK) cells?

Killing tumor cells and virus-infected cells Signup and view all the answers

Which receptors are expressed by Natural Killer (NK) cells that are important for their function?

CD16 and CD56 Signup and view all the answers

What is the significance of MHC class I molecules in the regulation of NK cell activity?

They provide inhibitory signals Signup and view all the answers

How do activating receptors on NK cells facilitate their cytotoxic function?

By detecting ill-defined molecules on cells Signup and view all the answers

What is a characteristic of MHC molecules in humans?

They are referred to as human leukocyte antigens (HLA) Signup and view all the answers

Which scenario would likely lead to increased expression of the activating receptor NKG2D on NK cells?

Viral infections Signup and view all the answers

What challenge do polymorphic MHC genes pose in organ transplantation?

They lead to mismatched tissue compatibility Signup and view all the answers

What percentage of peripheral blood lymphocytes are Natural Killer (NK) cells?

5% - 10% Signup and view all the answers

Which domains form the antigen binding groove in class II MHC molecules?

α1 and β1 domains Signup and view all the answers

What type of antigens do class II MHC molecules primarily present?

Exogenous antigens Signup and view all the answers

Which T cell type is recognized by the peptide fragments presented by class II MHC molecules?

CD4+ Helper T cells Signup and view all the answers

Which domain of class II MHC has a binding site for CD4?

β2 domain Signup and view all the answers

What factor can induce the expression of class II MHC on various cell types?

IFN-γ Signup and view all the answers

Which condition is associated with HLA-B27?

Ankylosing spondylitis Signup and view all the answers

What is required for the activation of naive T lymphocytes?

Antigen and co-stimulators Signup and view all the answers

What types of cells can CD4+ T cells differentiate into after activation?

TH1, TH2, and TH17 Signup and view all the answers

What role do TH2 cells play in the immune response to allergens?

They release cytokines like IL-4, IL-5, and IL-13. Signup and view all the answers

What is the primary function of IL-4 in the allergic response?

Promotes class switching to IgE in B-lymphocytes. Signup and view all the answers

What initiates the activation of sensitized mast cells during an allergic reaction?

Crosslinking of IgE Fc receptors by multivalent allergens. Signup and view all the answers

What happens when mast cells are sensitized and subsequently exposed to the same antigen?

They release mediators through degranulation. Signup and view all the answers

Which of the following is NOT a stimulus that can activate sensitized mast cells?

Infection with a virus. Signup and view all the answers

What mediators are released during mast cell degranulation?

Both primary and secondary mediators. Signup and view all the answers

What type of receptors do mast cells express to bind IgE antibodies?

FcεRI. Signup and view all the answers

What is a characteristic of non-atopic allergies?

They may be triggered by physical stimuli like temperature extremes. Signup and view all the answers

Study Notes

Immune System

The immune system protects against infection, mutant cells, tumour cells, and foreign substances.
Immune deficiencies lead to increased susceptibility to infections and diseases.
The immune system can also cause tissue damage and disease, such as hypersensitivity reactions and autoimmune diseases.

Antigens & Antibodies

Antigens are substances, usually proteins, that stimulate antibody production.
A hapten is a small molecule that requires a carrier molecule to elicit an immune response.
Antibodies (immunoglobulins) are proteins produced in response to antigenic stimulation.
There are five classes of immunoglobulins: IgG, IgA, IgM, IgE, and IgD.

Immune Response Mechanisms

The immune system can be divided into innate (natural) and adaptive (acquired) immunity.

Innate Immunity

Provides the first line of defense against pathogens.
Components include:
- Epithelia (skin, GIT, respiratory tract): act as mechanical barriers.
- Phagocytes (monocytes, macrophages, neutrophils): engulf and destroy pathogens.
- Dendritic cells: present antigens to adaptive immune cells.
- Natural killer cells: kill infected cells without prior sensitization.
- Innate lymphoid cells: contribute to early defense against microbes.
- Soluble proteins (complement, C-reactive protein, mannose-binding lectin): enhance immune responses.
Innate immunity mechanisms:
- Triggers inflammation by releasing cytokines, complement products, and mediators.
- Releases type I interferons: activate enzymes that degrade viral nucleic acids and inhibit viral replication.
- Stimulates a stronger adaptive immune response.
Innate immunity lacks memory and fine antigen specificity, unlike adaptive immunity.

Adaptive Immunity

Mediated by lymphocytes and their products (antibodies, cytokines), antigen-presenting cells (macrophages, dendritic cells, B-cells), mast cells/basophils, and eosinophils.
Divided into:
- Humoral immunity: mediated by B lymphocytes and protects against extracellular microbes and toxins.
- Cell-mediated immunity: mediated by T lymphocytes and protects against intracellular microbes.

Lymphoid Organs

Categorized into primary and secondary lymphoid organs.
Primary (generative):
- Bone marrow: site of B cell development.
- Thymus: site of T cell development.
Secondary (peripheral):
- Lymph nodes, spleen, mucosa-associated lymphoid tissue (MALT) of GIT and respiratory tract.

Natural Killer (NK) Cells

Also known as large granular lymphocytes due to cytoplasmic granules.
Make up 5-10% of peripheral blood lymphocytes.
Do not express T cell receptors (TCRs) or immunoglobulins.
Inherently capable of killing tumor cells and virus-infected cells without prior sensitization (innate immunity).
Express CD16 and CD56 markers.
CD16 is an Fc receptor for IgG, enabling antibody-dependent cell-mediated cytotoxicity (ADCC).
Function is regulated by the balance between activating and inhibitory receptors.
Activating receptors recognize molecules on target cells and stimulate killing.
Inhibitory receptors recognize MHC class I molecules and suppress killing.
NKG2D is an activating receptor whose expression increases with viral infections and neoplastic transformation, both of which also decrease MHC class I expression.

Major Histocompatibility Complex (MHC)

MHC molecules are essential for T cell recognition of antigens.
They display peptide fragments of protein antigens for recognition by antigen-specific T cells.
Determine tissue compatibility between individuals.
Also known as human leukocyte antigens (HLA).
Genes encoding MHC molecules are located on chromosome 6.
MHC genes are polymorphic, meaning there are many alleles in the population.
This allelic variation poses a challenge in organ transplantation.

MHC Class I Molecules

Expressed by all nucleated cells.
Present endogenous antigens, such as viral proteins, to CD8+ cytotoxic T lymphocytes (CTLs).
Composed of a heavy α chain and a light β2-microglobulin chain.
The α1 and α2 domains form the antigen-binding groove.
The α3 domain binds to CD8 on CTLs.

MHC Class II Molecules

Expressed by antigen-presenting cells (macrophages, dendritic cells, B cells).
Present exogenous antigens (proteins taken up from the environment) to CD4+ helper T lymphocytes.
Composed of an α chain and a β chain.
The α1 and β1 domains form the antigen-binding groove.
The β2 domain binds to CD4 on helper T cells.

HLA and Disease

Certain HLA alleles are associated with specific diseases:
- Ankylosing spondylitis and HLA-B27: increased risk of developing the disease.
- Inherited errors of metabolism (e.g., 21-hydroxylase deficiency) and specific HLA alleles (e.g., HLA-B47).
- Autoimmune diseases (e.g., endocrinopathies) and specific HLA-DR locus alleles.

Activation of T Lymphocytes and Elimination of Intracellular Microbes (Cell-Mediated Immunity)

Naive T lymphocytes are activated in peripheral lymphoid organs by antigens and co-stimulators (CD28 on T cells and B7 proteins on APCs).
Activated T cells proliferate and differentiate into effector and memory cells.
Effector cells migrate to sites of antigen presence.
CD4+ T cells differentiate into various effector cells (TH1, TH2, TH17) that secrete different cytokines with specific functions.

Type I Hypersensitivity Reactions

Allergic reactions mediated by IgE antibodies.
Triggered by allergens, which elicit strong TH2 responses.
Involve the following steps:
- Sensitization: Initial exposure to allergen leads to production of IgE antibodies.
- Mast cell sensitization: IgE antibodies bind to FcεRI receptors on mast cells.
- Re-exposure: Subsequent exposure to allergen crosslinks IgE on mast cells.
- Mast cell degranulation: Release of preformed mediators (histamine, proteases) and synthesis of secondary mediators (leukotrienes, prostaglandins).
- Clinical manifestations: Immediate hypersensitivity reactions due to mediator release.

Mast Cells

Marrow-derived cells found in subepithelial tissues.
Contain granules with various mediators.
Express FcεRI receptors, which bind IgE.
Sensitized mast cells release mediators upon allergen exposure.
Mediator release triggers clinical features of immediate hypersensitivity reactions.

Type I Hypersensitivity Reaction Triggers

Multivalent allergens crosslinking IgE on mast cells.
Other stimuli: C3a and C5a anaphylatoxins, IL-8, bee sting, drugs (mellitin, codeine), physical stimuli (temperature extremes).

Mediator Release

Degranulation of mast cells releases preformed mediators (primary mediators).
De novo synthesis and release of secondary mediators.

Primary Mediators

Preformed and stored in granules:
- Histamine: causes vasodilation, increased vascular permeability, bronchoconstriction.
- Proteases: degrade tissue proteins, contribute to inflammation.

Secondary Mediators

Synthesized upon mast cell activation:
- Leukotrienes: potent bronchoconstrictors, increase vascular permeability.
- Prostaglandins: cause vasodilation, increase vascular permeability.
Contribute to the development of late-phase allergic reactions.

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Description

This quiz explores the fundamentals of the immune system, including its role in protecting against infections and diseases. Learn about key concepts such as antigens, antibodies, and the mechanisms of innate and adaptive immunity. Test your knowledge on the complexities of immune responses and related conditions.