6.4.1 Antigens, antibodies and immune cells Quiz: Pearson Edexcel Biology A (Salters-Nuffield), Unit 6

The 20 questions

1. An antigen is best defined as:

   • A type of white blood cell that engulfs pathogens and digests them within lysosomes
   • A molecule, usually a protein or glycoprotein
   • A protein produced by B cells that circulates in the blood and destroys every pathogen it touches
   • A chemical released by mast cells that dilates blood vessels during an inflammatory response

2. Antibodies are produced by:

   • Neutrophils, which are short-lived phagocytes that move quickly to the site of infection
   • Plasma cells, which are differentiated B cells that secrete large quantities of antibody
   • Red blood cells, which carry antibodies around the body bound to their membrane proteins
   • Macrophages, which engulf pathogens and digest them with enzymes in their lysosomes

3. The structure of an antibody molecule typically consists of:

   • A single chain of protein folded into a spherical capsid that encloses nucleic acid
   • Two lipid layers forming an envelope around a central core of nucleic acid and proteins
   • Four identical chains of peptidoglycan joined by glycosidic bonds to form a rigid wall
   • Two heavy chains and two light chains joined by disulfide bonds

4. The variable region of an antibody is important because it:

   • Attaches the antibody to the surface of macrophages, which then digest the antibody
   • Activates the complement cascade directly, without binding to any antigen first
   • Forms the antigen-binding site, which has a shape complementary to a specific antigen
   • Carries the genetic code for the constant region of the antibody and so controls its class

5. Macrophages and antigen-presenting cells contribute to the immune response by:

   • Engulfing pathogens and displaying fragments of their antigens on the cell surface to activate helper T cells
   • Releasing histamine that causes blood vessels to dilate and allow lymphocytes into the tissue
   • Binding directly to pathogens and killing them by releasing perforin into the infected cell
   • Producing large quantities of antibodies that are secreted into the blood to neutralise pathogens

6. Which function of antibodies involves clumping pathogens together so they are easier to phagocytose?

   • Opsonisation of the lysosomes, in which antibodies activate the digestive enzymes inside them
   • Lysis, in which antibodies bind to the phospholipid membrane of the host cell and break it
   • Neutralisation, in which antibodies bind to the active site of an enzyme and inhibit its action
   • Agglutination, in which antibodies bind several pathogens and form clumps that are easier to engulf

7. Which term describes the binding of antibodies to toxins so that the toxin cannot bind to host cells?

   • Lysis, in which the antibody breaks the toxin into small pieces that can be excreted
   • Neutralisation, in which the antibody blocks the site that the toxin needs to attach to cells
   • Agglutination, in which the antibody clumps toxins together into large inert aggregates
   • Opsonisation, in which the antibody coats the toxin so that phagocytes can engulf it

8. Antigen-presenting cells display antigen fragments on which molecules?

   • MHC class II molecules, which are recognised by helper T cells
   • Peptidoglycan chains, which are recognised by memory B cells in the lymph nodes
   • Surfactant proteins, which are recognised by B cells in the alveolar walls of the lung
   • Haemoglobin molecules, which are recognised by killer T cells in the blood

9. A patient produces antibodies that bind to a virus. Which property of the antibody makes this binding specific?

   • The shape of the variable region is complementary to a particular epitope on the viral antigen
   • The constant region is identical to the viral capsid, so it binds to any viral particle
   • The antibody is produced by every B cell in the body, so it binds to any antigen equally well
   • The antibody contains peptidoglycan, which binds only to viral envelope glycoproteins

10. An epitope is best described as:

    • The entire protein molecule that is produced by a plasma cell in response to infection
    • The molecule that carries oxygen to the tissues and is released by damaged red blood cells
    • The part of an antigen that is recognised by and binds to a particular antibody or receptor
    • The region of the cell membrane that controls the entry of antibodies into the cell

11. Which cell is responsible for presenting antigen to helper T cells as part of the adaptive response?

    • Erythrocytes, which carry the antigens to the lymph nodes on their membrane surface
    • Mast cells, which release histamine but do not process antigen for presentation
    • Macrophages and dendritic cells, which act as antigen-presenting cells
    • Neutrophils, which release digestive enzymes that destroy pathogens in the blood

12. Which feature distinguishes antibodies from antigens?

    • Antibodies are made only in bacteria, whereas antigens are made only by human cells
    • Antibodies are proteins made by the immune system to bind antigens, whereas antigens are molecules that trigger the response
    • Antibodies are foreign molecules that trigger the response, whereas antigens are made by the host
    • Antibodies are carbohydrates with no protein content, whereas antigens are always lipids

13. Why does a single antigen often elicit antibodies against several different epitopes?

    • Because each antigen is made of a single identical epitope repeated many times on the surface
    • Because antibodies can change their shape after binding so that they recognise every part of the antigen
    • Because a large antigen such as a protein has multiple distinct regions, each capable of being recognised by different B cells
    • Because each epitope is made of nucleic acid, which can be recognised only by memory B cells

14. Which statement about antibody classes is correct?

    • Antibodies differ in their variable regions only, and their constant regions are always identical
    • Antibodies differ only in their colour, which indicates whether they are produced in the blood
    • Antibodies differ only in the number of light chains, which is always one for all classes
    • Antibodies differ in their constant regions

15. Opsonisation is best described as:

    • Coating of a pathogen by antibodies so that phagocytes bind to and engulf it more easily
    • Coating of a virus by lysozyme so that its capsid is broken down before it can enter cells
    • Coating of a red blood cell by antibodies so that it becomes unable to carry oxygen
    • Coating of a phagocyte by antigens so that it can present them to helper T cells

16. A researcher finds that a serum sample neutralises a toxin in vitro. Which best explains this result?

    • Red blood cells in the serum engulfed the toxin, which was then digested in their lysosomes
    • Histamine in the serum dissolved the toxin into harmless fragments that could be excreted
    • Lysozyme in the serum broke down the toxin's peptidoglycan wall, destroying its structure
    • Antibodies in the serum bound the toxin's active sites, preventing it from attaching to target cells

17. Which statement evaluates the claim that antibodies destroy pathogens directly?

    • The claim is accurate, since antibodies kill pathogens by releasing perforin into their membranes
    • The claim is inaccurate, since antibodies have no role in the immune response at any stage
    • The claim is accurate, since every antibody contains enzymes that digest pathogens on contact
    • The claim is partly inaccurate, since antibodies mainly tag, neutralise or clump pathogens for destruction by other immune cells

18. A pathogen expresses a new antigen that the immune system has not previously encountered. Which best describes the initial response?

    • Slow, because only a few B cells with matching receptors are present and must be clonally selected and expanded
    • Immediate and large, because all B cells already produce antibodies against every possible antigen
    • Immediate and large, because the constant region of antibodies recognises all new antigens
    • Absent, because antibodies cannot be made against antigens that have not previously been seen

19. Which statement describes the role of helper T cells in relation to antibody production?

    • They inhibit the division of B cells, which prevents antibody production from becoming excessive
    • They engulf antigens and digest them in their lysosomes before any B cell can be activated
    • They are activated by antigen presented on MHC class II
    • They produce antibodies directly and secrete them into the blood as soon as they are activated

20. Which of the following is an example of an antigen found on the surface of a pathogen rather than a human cell?

    • The peptidoglycan layer that is found in the cell walls of all human cells and tissues
    • A glycoprotein spike on the envelope of an enveloped virus
    • The haemoglobin molecule that is found on the membrane of every mature red blood cell
    • The MHC class I molecule that is found on the surface of every nucleated human cell

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