Intro to Immunology 3&4 (B,T cells)

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Questions and Answers

Which type of lymphocytes are responsible for humoral immune responses mediated by antibody molecules secreted by plasma cells?

  • CD8+ T cells
  • T lymphocytes
  • CD4+ T cells
  • B lymphocytes (correct)

What is the key role of B cells in the immune system?

  • Defence against extracellular pathogens (correct)
  • Defence against intracellular pathogens
  • Regulating the immune system
  • Killing virally infected body cells

Which type of T cells are responsible for killing virally infected body cells?

  • B lymphocytes
  • Helper T cells
  • CD4+ T cells
  • CD8+ T cells (correct)

What is the function of CD4+ T cells?

<p>Regulating the immune system (D)</p> Signup and view all the answers

How do B cells recognize and bind to antigens?

<p>Using membrane-bound antibodies as receptors (D)</p> Signup and view all the answers

How many copies of a specific BCR antibody do B cells express?

<p>50,000 copies (B)</p> Signup and view all the answers

What are antibodies composed of?

<p>Two light chains and two heavy chains (D)</p> Signup and view all the answers

What do antibodies bind to?

<p>Antigens (B)</p> Signup and view all the answers

Which region of the TCR forms the antigen binding site and is unique to each individual T cell?

<p>Hyper variable region (C)</p> Signup and view all the answers

What is the function of MHC/HLA molecules?

<p>Express peptide antigens (C)</p> Signup and view all the answers

How many copies of a single antigen receptor does a T cell express?

<p>Thousands (D)</p> Signup and view all the answers

What is the role of TCRs in recognizing antigens?

<p>Bind to MHC molecules (C)</p> Signup and view all the answers

What is another name for Major Histocompatibility Complex (MHC) molecules?

<p>HLA molecules (A)</p> Signup and view all the answers

What type of antigens can T cells recognize?

<p>Peptide antigens (A)</p> Signup and view all the answers

What is the function of MHC molecules in relation to T cells?

<p>Bind to peptide antigens (C)</p> Signup and view all the answers

What is the role of the hyper variable region in the TCR?

<p>Form the antigen binding site (D)</p> Signup and view all the answers

What is the unique feature of each T cell's TCR?

<p>Hyper variable region (A)</p> Signup and view all the answers

What is the role of MHC molecules in relation to T cells?

<p>MHC molecules present peptide antigens to T cells.</p> Signup and view all the answers

What is the function of TCRs in recognizing antigens?

<p>TCRs bind to peptide antigens presented by MHC molecules.</p> Signup and view all the answers

What type of antigens can T cells recognize?

<p>T cells can recognize peptide antigens.</p> Signup and view all the answers

What is the key role of B cells in the immune system?

<p>B cells produce antibodies and mediate humoral immune responses.</p> Signup and view all the answers

What is the unique feature of each T cell's TCR?

<p>The hyper variable region formed by the tips of the alpha and beta TCR chains is unique to each T cell.</p> Signup and view all the answers

What do antibodies bind to?

<p>Antibodies bind to antigens.</p> Signup and view all the answers

What is the function of CD4+ T cells?

<p>CD4+ T cells help coordinate immune responses and activate other immune cells.</p> Signup and view all the answers

What is another name for Major Histocompatibility Complex (MHC) molecules?

<p>MHC molecules are also referred to as HLA (Human Leukocyte Antigens).</p> Signup and view all the answers

What is the role of B lymphocytes in the adaptive immune system?

<p>B lymphocytes have an important function in the adaptive immune system. They are responsible for humoral immune responses which are mediated by antibody molecules secreted by plasma cells.</p> Signup and view all the answers

What is the role of T lymphocytes in the immune system?

<p>T lymphocytes are responsible for cellular immune responses and thus have a key role in defence against intracellular pathogens.</p> Signup and view all the answers

What are the two types of T lymphocytes and what are their functions?

<p>The two types of T lymphocytes are CD4+ T cells (Helper T cells) and CD8+ T cells (Cytotoxic T cells). CD4+ T cells are the key regulators of the entire immune system, sending signals for cytokines to perform their functions. CD8+ T cells kill virally infected body cells.</p> Signup and view all the answers

What is the process by which B and T cells mature to distinguish self from non-self?

<p>B and T cells mature to distinguish self from non-self through a process called acquired immune tolerance. If they react to self antigens, they are usually destroyed or inactivated.</p> Signup and view all the answers

How do acquired immune cells detect invading pathogens?

<p>Acquired immune cells use specific recognition to detect invading pathogens. The pathogens have unique structures called antigens that are recognized by the adaptive leukocytes.</p> Signup and view all the answers

What are antibodies and what is their structure?

<p>Antibodies are proteins that bind to one specific antigen. They are a complex of four polypeptide chains, which include 2 light chains and 2 heavy chains.</p> Signup and view all the answers

How do B cells recognize and bind to antigens?

<p>B cells use membrane-bound antibodies as receptors to recognize and bind to membrane-associated or soluble antigens.</p> Signup and view all the answers

What are the two types of antibodies that are the most abundant in the plasma?

<p>IgG and IgA</p> Signup and view all the answers

Where do B cells mature in the body?

<p>Bone marrow</p> Signup and view all the answers

Where do B and T lymphocytes exist in their inactive state?

<p>B and T cells normally circulate around their primary lymphoid tissue (site of development - bone marrow/thymus) in their inactive form</p> Signup and view all the answers

Where and how do B and T cells get activated?

<p>Antigen presentation activates the B cells and T cells in secondary lymphoid tissue (e.g. lymph nodes, spleen, MALT)</p> Signup and view all the answers

What are the two processes that take place when B cells encounter antigen?

<p>B cell activation and the eventual production of high affinity antibodies.</p> Signup and view all the answers

Describe the processes of B cell activation followed by production of antibodies when it encounters antigens?

<ol> <li>Membrane-bound antibodies on the B cells bind to target antigen IgM (or IgD) within the B cell zone of lymph nodes</li> <li>B cells require 2 signals to become fully active and begin proliferation - the antigen and ‘helper signals’ e.g. from TH cells, PRR and PAMPs or from multiple antigens</li> <li>Once activated, they clonally proliferate and become either a plasma cell (antibodies) or a memory B cell - germinal centre response</li> <li>High affinity antibodies are generated - IgM (produced by plasma cells) → IgG (produced by B cells responding to certain antigens), assisted by TH cells</li> </ol> Signup and view all the answers

What are the two activation signals for non protein antigens?

<p>Signal 1- BCR and antigen complex Signal 2- T cell independent reaction therefore it requires PRR+PAMP complex. This produces low affinity antibodies and short lived plasma cells. Without the help of T cell response, it only provides a short lived response and no Memory cells.</p> Signup and view all the answers

What are the two activation signals in case of a protein antigen?

<p>Signal 1- BCR binding to antigen Signal 2- For T-dependent antigens, which are typically proteins, an additional co-stimulatory signal is required for full B cell activation. After BCR engagement with the antigen, the B cell presents processed antigenic peptides on its surface using (MHC II) molecules. Interaction between the B cell and a CD4+ helper T cell occurs when the T cell recognizes the antigen-MHC II complex on the B cell surface. This interaction provides the second signal.</p> Signup and view all the answers

What are the two functions antibodies use to help kill and eliminate antigens/pathogens?

<p>1)Recognition function- Binding to antigen mediated by variable region sites. 2)Effector function- Clearance mechanisms mediated interaction of the heavy chain constant region with effector molecules.</p> Signup and view all the answers

What are the functions of IgM antibody?

<p>B cell activation, agglutination (immune complex formation - enhances phagocytosis), complement system activation through classical pathway</p> Signup and view all the answers

What is the most abundant antibody in the plasma?

<p>IgG</p> Signup and view all the answers

What is the second most abundant antibody?

<p>IgA</p> Signup and view all the answers

What is the function of IgE antibody?

<p>Produced in allergic response</p> Signup and view all the answers

What is the function of the IgA antibody?

<ul> <li>helps with neutralization of pathogens (membrane bound form)</li> <li>Dimer in secretory fluids (e.g. colostrum) <ul> <li>Neonatal defense - protects GI tract of neonates</li> </ul> </li> </ul> Signup and view all the answers

What is the order of abundance of antibodies (Ig) in the blood plasma?

<p>IgG&gt;IgA&gt;IgM&gt;IgD&gt;IgE</p> Signup and view all the answers

What is the function of IgD antibodies?

<p>It plays a role in B cell activation.</p> Signup and view all the answers

What is the function of IgG antibodies?

<p>foetal immunity (placental transfer), complement activation, NK cell activation, neutralisation, opsonisation, agglutination</p> Signup and view all the answers

What is the process of Natural Killer cell activation in innate immune response?

<p>Innate immune response- Intracellular pathogens infect host cells. The PAMPs stimulate the secretion of Interferons alpha and beta (IFN alpha/beta). These interferons activate the natural killer cells which perform the killing of infected host cells and produce pro inflammatory mediators.</p> Signup and view all the answers

What is the process of Natural killer cell activation in the adaptive (humoral) immune response of the body?

<p>The antigen bound IgG is a good activator of natural killer cells which then performs the killing of infected host cells by apoptosis and produces pro inflammatory mediators.</p> Signup and view all the answers

What are the functions of CD4+ Helper T cells?

<p>CD4+ Helper T cells activate B cells and stimulate production of memory B cells.</p> Signup and view all the answers

What is the function of CD8+ Killer T cells?

<p>CD8+ Killer T cells kill infected cells via perforin, granzymes, and granulysin.</p> Signup and view all the answers

What is the role of Regulatory T cells?

<p>Regulatory T cells suppress lymphocyte activity.</p> Signup and view all the answers

What are the two types of MHC molecules and which T cells do they present peptide antigens to?

<p>Class I MHC: expressed on all nucleated cells, present peptide antigens to CD8+ killer T cells; Class II MHC: expressed only on antigen presenting cells (e.g. dendrites, macrophages), present peptide antigens to CD4+ helper T cells</p> Signup and view all the answers

What is the role of MHC molecules in relation to T cells?

<p>MHC molecules present peptide antigens to T cells, allowing T cells to recognize and respond to specific antigens</p> Signup and view all the answers

What are the two types of T cells that can recognize peptide antigens presented by MHC molecules and what are their functions?

<p>CD8+ killer T cells: recognize peptide antigens presented by Class I MHC molecules and destroy infected cells; CD4+ helper T cells: recognize peptide antigens presented by Class II MHC molecules and help coordinate immune responses</p> Signup and view all the answers

What are the two types of MHC molecules and which T cell subsets do they present peptide antigens to?

<p>Class I MHC: presented to CD8+ killer T cells, Class II MHC: presented to CD4+ helper T cells</p> Signup and view all the answers

Where are Class I MHC molecules expressed and what type of cells do they present peptide antigens to?

<p>Expressed on all nucleated cells, present peptide antigens to CD8+ killer T cells</p> Signup and view all the answers

Where are Class II MHC molecules expressed and what type of cells do they present peptide antigens to?

<p>Expressed only on antigen presenting cells (e.g. dendritic cells, macrophages), present peptide antigens to CD4+ helper T cells</p> Signup and view all the answers

How does the dendritic cell get prepared for T cell differentiation?

<ol> <li>Dendritic cells recognise and phagocytose antigenic debris</li> <li>In the presence of pro-inflammatory mediators (e.g. TNF⍺), the dendritic cells mature and increase expression of stimulatory molecules on their surface</li> <li>Dendritic cells phagocytose the pathogenic antigens, break the antigens down into short peptides and load them onto MHC II molecules</li> <li>MHC II molecules transported to cell surface <ol> <li>At the same time, the maturing dendritic cells migrate into the lymph nodes via the afferent lymphatic vessels</li> </ol> </li> <li>Co-stimulatory molecules enable T cell to respond to antigen and fully differentiate</li> </ol> Signup and view all the answers

How does the naive/resting CD4+ T cell proliferate into the T helper (TH0) cell?

<p>The dendritic cell expresses the MHC II peptide, it forms a complex with the antigen activated CD4+ T cell. The CD4+ T cell secretes T cell growth factors called IL-2 which are autocrine mediated. These IL-2 growth factors promote T cell proliferation and differentiation.</p> Signup and view all the answers

What is the function of TH1 cells?

<p>TH1 cells: secrete pro-inflammatory cytokines (IL2, IFN𝛾), which stimulates production of Reactive Oxygen Species (macrophage-mediated)-(switch on expression of NADPH oxidase genes). This allows the macrophages to kill the bacteria much more effectively.</p> Signup and view all the answers

What is the function of TH2 cells?

<p>TH2 cells: secrete mainly IL4, IL5 and IL6 which promote B cell proliferation and induces antibody production. Th2 cells stimulate and recruit specialized subsets of immune cells, such as eosinophils and basophils, to the site of infection or in response to allergens or toxins.</p> Signup and view all the answers

What is the function of TFH cells (T follicular helper cells)?

<p>FH cells: stimulated by presented antigen peptides + MHC II molecules on B cell, then stimulate the B cells to proliferate and differentiate (plasma + memory) by secreting IL4 and IL21</p> Signup and view all the answers

How does CD8+ T cell differentiation take place?

<p>IL-2 (provided by TH cells) promotes differentiation and proliferation of antigen activated (via MHC I) CD8+ cells into cytotoxic T lymphocytes (CTLs)</p> Signup and view all the answers

What is the mechanism of killing by Cytotoxic T cells?

<ul> <li>CTLs migrate out of secondary nodes and enter site of infection (via transendothelial migration) and then kill virally infected host cells by recognizing pathogenic antigens attached to MHC class I <ul> <li>Also involved in killing cancer cells</li> </ul> </li> </ul> Signup and view all the answers

What do Cytotoxic T cells secrete to kill infected host cells?

<p>There are proteins in lytic granules of cytotoxic T cells:-Perforin, Granzymes and Granulysin. These substances are secreted in order to kill in a similar mechanism to NK cells.</p> Signup and view all the answers

What is the action of Perforin?

<p>It is a cytotoxic granule secreted by the T cell polymerises to form a pore in the target membrane.</p> Signup and view all the answers

What is the action of Granzymes?

<p>Granzymes are secreted by cytotoxic T cells. They are serine proteases, which activate apoptosis once in the cytoplasm of the target cell.</p> Signup and view all the answers

What is the action of Granulysin?

<p>Granulysin is a substance secreted by the Cytotoxic T cells which induces apoptosis in infected host cells.</p> Signup and view all the answers

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Study Notes

B Lymphocytes

  • Crucial for the adaptive immune system, mediating humoral immune responses through antibodies produced by plasma cells.
  • Defend against extracellular pathogens by producing antibodies that attack circulating pathogens in blood and lymph.
  • Upon activation, B cells clonally proliferate into plasma cells or memory B cells, generating high-affinity antibodies.

T Lymphocytes

  • Responsible for cellular immune responses, protect against intracellular pathogens.
  • CD4+ T Cells (Helper T cells): Key regulators that send signals for cytokine production to activate other immune cells.
  • CD8+ T Cells (Cytotoxic T cells): Kill virally infected cells directly.
  • Mature in the thymus, developing the ability to distinguish between self and non-self, leading to destruction/inactivation of self-reactive cells.

Antibody Structure and Function

  • Antibodies consist of four polypeptide chains: two light chains and two heavy chains.
  • Each antibody has a unique variable region for binding specific antigens and conserved constant regions.
  • B cells express 50,000 copies of a single B cell receptor (BCR) that recognizes soluble or membrane-bound antigens.
  • Antibodies are found in two forms: membrane-bound (on B cell surface) and soluble (produced by plasma cells).

T Cell Receptor (TCR) Specificity

  • T cells solely recognize peptide antigens; each T cell expresses thousands of a single antigen receptor.
  • Unique hyper-variable region on TCRs allows binding to specific peptide antigens presented by MHC molecules.
  • MHC molecules (also known as HLA) present various peptide antigens to T cells for recognition.

Antigen Presentation and Activation

  • B cell activation occurs in secondary lymphoid tissue (e.g., lymph nodes, spleen) following binding of membrane-bound antibodies to antigens.
  • T cells require two signals for full activation: specific antigen recognition and helper signals from other immune cells.
  • Dendritic cells play a vital role in antigen presentation, maturing and loading peptides onto MHC II for T cell recognition.

Major Histocompatibility Complex (MHC)

  • MHC Class I: Present on all nucleated cells, presenting antigens to CD8+ T cells.
  • MHC Class II: Found solely on antigen presenting cells like dendritic cells and macrophages, presenting antigens to CD4+ T cells.

Antibody Classes

  • IgM: First antibody produced during an infection, exists as a monomer on B cells and as a pentamer when secreted; functions in agglutination and complement activation.
  • IgG: Most abundant antibody in plasma, involved in fetal immunity, neutralization, and complement activation.
  • IgA: Second most abundant, found as a monomer in serum and a dimer in secretory fluids, plays a role in mucosal immunity.
  • IgD: Least abundant; primarily involved in B cell activation.
  • IgE: Least common; associated with allergic responses.

T Cell Subtypes and Functions

  • Helper T Cells: Secrete cytokines (e.g., IL-2) to help activate other immune cells and stimulate B cell memory production.
  • Cytotoxic T Cells: Kill infected host cells via perforin and granzymes, also can target cancer cells.
  • Regulatory T Cells: Suppress autoimmune responses and maintain immune balance.
  • Memory T Cells: Formed for quick response during subsequent exposures to pathogens.

Summary of Immune Defense Against Pathogens

  • Phagocytes target bacteria, T lymphocytes combat viruses, and B lymphocytes produce antibodies against fungi and protozoa.
  • Specific immune components interact with pathogens to elicit appropriate immune responses based on the type and characteristics of the invading organism.

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