Immune System Overview and Cells

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Questions and Answers

Which component of innate immunity is primarily responsible for phagocytosis?

B lymphocytes

Neutrophils and macrophages (correct)

Antibodies

Natural killer cells

What is the primary function of CD4 T-lymphocytes?

Bind to class I MHC molecules

Bind to class II MHC molecules (correct)

Produce antibodies

Facilitate delayed hypersensitivity reactions

Which cytokine is secreted by T-helper-1 (TH1) cells?

Interferon-γ (IFN-γ) (correct)

IL-5

IL-13

IL-4

What essential structural component is part of the T cell receptor (TCR)?

α and β polypeptide chains Signup and view all the answers

How does the CD4:CD8 ratio typically present in healthy individuals?

2:1 Signup and view all the answers

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

Humoral immunity Signup and view all the answers

The somatic rearrangement of TCR genes is crucial for what purpose?

Generating TCR diversity Signup and view all the answers

Which cytokines are produced by T-helper-2 (TH2) cells?

IL-4, IL-5, and IL-13 Signup and view all the answers

Which aspect of the immune system is involved in recognizing nonmicrobial substances?

Adaptive immunity Signup and view all the answers

Natural killer (NK) cells are primarily involved in what type of immunity?

Innate immunity Signup and view all the answers

What is the main function of CD8+ T cells?

Killing other cells and secreting cytokines Signup and view all the answers

Which of the following best describes the activation of T cells?

Two signals are required: TCR engagement and CD28 interaction Signup and view all the answers

How do B cells contribute to the immune response following antigen stimulation?

They transform into plasma cells producing antibodies Signup and view all the answers

Which immunoglobulin is primarily found on the surface of all B cells?

IgM Signup and view all the answers

What role do macrophages play in the immune response?

They present antigens and produce cytokines Signup and view all the answers

Which type of cells are known for their efficiency in antigen presentation?

Dendritic cells Signup and view all the answers

What is the primary function of CD40 on B cells?

To interact with activated T-lymphocytes CD154 Signup and view all the answers

What distinguishes follicular dendritic cells from other dendritic cells?

They contain antibodies on their cell surfaces Signup and view all the answers

Which of the following immunoglobulins is secreted in the least amount in serum?

IgE Signup and view all the answers

What characterizes the process of somatic rearrangement of immunoglobulin genes in B cells?

It results in unique antigen specificity Signup and view all the answers

What is the primary role of natural killer cells in the immune system?

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

Which of the following correctly identifies the markers used to identify NK cells?

CD16 and CD56 Signup and view all the answers

How do class I MHC molecules function in relation to CD8+ T cells?

They display peptides derived from intracellular proteins. Signup and view all the answers

What type of receptors do NK cells express to interact with class I MHC molecules?

Both activator and inhibitory receptors Signup and view all the answers

What is a primary function of the molecules encoded by class II MHC genes?

Presenting exogenous antigens to CD4+ T cells Signup and view all the answers

What distinguishes CD8+ T cells in terms of their antigen recognition?

They require MHC class I for recognition of peptides. Signup and view all the answers

What percentage of peripheral blood lymphocytes do NK cells typically comprise?

10-15% Signup and view all the answers

Which of the following correctly describes the mechanism of antibody-dependent cell-mediated cytotoxicity?

Recognition of IgG-coated cells via the Fc receptor CD16 Signup and view all the answers

Which statement about class III MHC genes is true?

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

Where are the genes that encode MHC molecules located?

On chromosome 6 Signup and view all the answers

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

Vasodilatation and exudation Signup and view all the answers

Which mechanism is NOT involved in Type II hypersensitivity reactions?

Cell-mediated cytotoxicity Signup and view all the answers

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

Systemic lupus erythematosus Signup and view all the answers

Which of these best describes the late phase of a Type I hypersensitivity reaction?

Lasts for days due to leukocyte influx Signup and view all the answers

In Type IV hypersensitivity, which cell type is primarily responsible for delayed-type hypersensitivity reactions?

CD4+ T cells Signup and view all the answers

What type of hypersensitivity is primarily involved in tissue damage through circulating immune complexes?

Type III hypersensitivity Signup and view all the answers

Which condition is NOT typically associated with Type II hypersensitivity reactions?

Poststreptococcal glomerulonephritis Signup and view all the answers

Which of the following is a primary mechanism of Type II hypersensitivity leading to direct damage?

Opsonization and phagocytosis Signup and view all the answers

Which of the following represents an incorrect association between hypersensitivity types and their characteristics?

Type II - Immune complex deposition Signup and view all the answers

Which statement about Type IV hypersensitivity is accurate?

It involves mechanisms of graft rejection. Signup and view all the answers

Study Notes

Immune System Overview
The immune system is a complex network that plays a critical role in protecting the body from pathogens and foreign substances. Within this system, there are two fundamental types of immune responses: innate and adaptive.
Innate immunity refers to the defense mechanisms that are part of the body's first line of defense and are present even before any exposure to a pathogen. These innate barriers include a variety of physiological and cellular components such as epithelial barriers, which form a physical barrier to pathogen entry; phagocytic cells, which engulf and destroy pathogens; natural killer (NK) cells that target and kill infected or tumor cells; and complement proteins, which enhance the ability of antibodies and phagocytic cells to clear microbes and damaged cells.
Adaptive immunity, in contrast, is a more specific response that develops upon exposure to infectious agents and can recognize unique non-microbial substances known as antigens. This type of immunity evolves over time and is characterized by its ability to remember previous infections, leading to a heightened response upon re-exposure.
- Adaptive immunity can be divided into two main types:
Cell-mediated immunity, involving T lymphocytes (which are derived from the thymus), primarily focuses on combating intracellular pathogens like viruses and some bacteria.
Humoral immunity, on the other hand, involves B lymphocytes (developing from the bone marrow) that produce antibodies to neutralize extracellular pathogens and toxins.
Immune Cells are the specialized cells that form the core of the immune response. The two main categories of immune cells include:
- T lymphocytes:
T lymphocytes constitute approximately 60-70% of the circulating lymphocytes in the bloodstream and reside mainly in the lymph nodes and spleen, where they can encounter antigens.
Each T cell possesses a uniquely structured T cell receptor (TCR) that is essential for recognizing specific antigens. This TCR is formed from two polypeptide chains, designated α and β.
- Additionally, the T cells express coreceptors known as CD4 and CD8.
CD4 coreceptors bind to class II MHC (Major Histocompatibility Complex) molecules found on antigen-presenting cells, facilitating helper T cell function. Conversely, CD8 coreceptors bind to class I MHC molecules, which are present on all nucleated cells, enabling cytotoxic functions.
- T helper (TH) cells can be further classified into two distinct subsets based on their cytokine production and functions:
TH1: This subset primarily produces cytokines such as IL-2 and IFN-γ, which play critical roles in delayed hypersensitivity reactions and in the activation of macrophages, thus enhancing the immune response against intracellular pathogens.
TH2: The second subset produces cytokines like IL-4, IL-5, and IL-13. These cytokines are crucial for stimulating B cell differentiation and promoting antibody production, while also enhancing eosinophil activation and combating parasitic infections.
CD8+ T cells: These are cytotoxic cells that can induce cell death in infected or malignant cells, thus playing a vital role in eliminating intracellular threats. They also have the ability to secrete various cytokines that modulate immune responses.
- The activation of T cells requires two essential signals:
First, there is TCR engagement with the MHC-antigen complex, where either CD4 or CD8 coreceptors assist in stabilizing this interaction.
Second, an important interaction occurs between the CD28 molecule expresses on T cells and CD80/CD86 ligands present on antigen-presenting cells; this second signal is crucial for full activation of T cells.
- B lymphocytes:
B lymphocytes represent about 10-20% of the circulating lymphocyte population and are likewise concentrated in lymph nodes and the spleen.
Upon encountering antigens, B cells undergo a process of activation and transformation into plasma cells, which are responsible for the synthesis and secretion of different classes of antibodies including IgG, IgM, IgA, and IgE. These antibodies are critical for identifying and neutralizing pathogens.
Monomeric IgM functions as an antigen receptor on the surface of B cells, effectively serving as the B cell receptor (BCR).
Activation of B cells involves a somatic rearrangement of their immunoglobulin genes, allowing for a diverse and specific response to a vast array of antigens.
In addition to BCRs, B lymphocytes also express other molecules like CD19, CD20, CD21, and CD40, which aid in their activation and help in signaling during immune responses.
- Macrophages:
Macrophages are versatile immune cells that play several crucial roles in the immune response. They present antigens to T cells through class II MHC molecules, thereby linking innate and adaptive immunity.
They produce a variety of cytokines that influence both T and B cell behavior, thereby modulating the overall immune response.
Macrophages also secrete toxic metabolites and proteolytic enzymes that are effective in lysing tumor cells and clearing pathogens.
These cells are particularly important in delayed hypersensitivity reactions as they can present persistent antigens and facilitate ongoing immune responses.
- Dendritic and Langerhans cells:
Dendritic cells possess unique dendritic cytoplasmic processes and have pronounced expression of class II MHC molecules, making them highly efficient antigen presenters.
Dendritic cells are typically located in lymphoid tissue, while Langerhans cells, which share similar functions, are primarily found in the epidermis.
While these cells are exceptional at presenting antigens to T cells, they exhibit relatively poor phagocytic activity compared to other immune cells.
Follicular dendritic cells found in germinal centers play a specialized role; they are adept at trapping antibodies and presenting them to B cells for further activation and maturation.
- Natural killer cells (NK cells):
Natural killer (NK) cells comprise about 10-15% of peripheral blood lymphocytes and are a key component of the innate immune system.
These cells do not express TCR or immunoglobulin, distinguishing them from T and B lymphocytes.
NK cells are able to kill tumor cells and cells infected with viruses without the need for prior sensitization, providing a rapid response to potential threats.
They can be identified by specific surface markers such as CD16 and CD56, which are used to classify their functional status as well as their developmental stage.
NK cells express both activating and inhibitory receptors that regulate their cytotoxic activity, ensuring that healthy cells are not harmed while targeting infected or malignant cells. They also secrete cytokines such as IFN-γ, which further modulate the immune response.
Major Histocompatibility Complex (MHC):
MHC molecules play a pivotal role in the immune system by presenting peptide fragments of foreign proteins to T cells, thereby facilitating the recognition of pathogens.
The MHC genes are located on chromosome 6 and are collectively referred to as the human leukocyte antigen (HLA) complex.
- There are three distinct classes of MHC molecules, each serving different functions:
Class I MHC: These molecules are expressed on the surface of all nucleated cells and are responsible for presenting peptides derived from intracellular proteins to CD8+ cytotoxic T cells. This is essential for the cellular immune response.
Class II MHC: These molecules are specifically expressed on professional antigen-presenting cells, such as macrophages, dendritic cells, and B cells. They present peptides derived from extracellular proteins to CD4+ helper T cells, playing a crucial role in orchestrating the immune response.
Class III MHC: This class encodes various components of the complement system, which aids in opsonization and clearance of pathogens, contributing to innate immune responses.
Hypersensitivity Reactions are immune reactions that can lead to inappropriate responses, resulting in diseases through four primary mechanisms:
Type I (immediate) hypersensitivity:
This hypersensitivity reaction develops rapidly within minutes of antigen exposure and is mediated primarily by mast cells and IgE antibodies.
The process can be divided into two distinct phases:
- The immediate phase: This involves vasodilation and increased vascular permeability, primarily driven by the release of vasoactive amines from activated mast cells. These changes lead to typical allergic symptoms such as swelling, redness, and itching.
- The late phase: This occurs several hours later (typically within 2-4 hours) and is characterized by an influx of leukocytes, including eosinophils and neutrophils, contributing to tissue inflammation and damage.
Type II hypersensitivity is characterized by the presence of antibodies that target cell surface or extracellular matrix antigens.
Destruction of the target cells or tissues occurs through several mechanisms, including:
- Opsonization, which promotes phagocytosis of target cells.
- Antibody-dependent cellular cytotoxicity (ADCC), wherein immune cells recognize and eliminate antibody-coated target cells.
- Antibody-mediated cellular dysfunction, leading to altered function of the target cells, often seen in autoimmune conditions.
Type III hypersensitivity involves immune complex-mediated reactions.
In this type, antigen-antibody complexes form in circulation and deposit in various tissues, eliciting a localized inflammatory response. Such activities can lead to tissue damage and are associated with diseases like systemic lupus erythematosus, rheumatoid arthritis, and more.
Type IV hypersensitivity is characterized by a cell-mediated immune response, primarily initiated by antigen-activated T lymphocytes.
This form of hypersensitivity includes delayed-type reactions, typically mediated by CD4+ T helper cells, as well as cytotoxic reactions that involve CD8+ cytotoxic T cells, contributing significantly to immune-mediated tissue injury.

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Description

This quiz covers the fundamental concepts of the immune system, including the distinction between innate and adaptive immunity. It also explores various immune cells, focusing on T lymphocytes and their functions in immune responses. Test your knowledge about these critical elements of immunology.