# 63 Summary of Immunity

1 Phagocytes and lymphocytes are the cells of the immune system.

2 The diagram shows how to recognise phagocytes and lymphocytes in microscope slides and photomicrographs of blood.





3 Phagocytes originate in the bone marrow and are produced there throughout life. There are two types: neutrophils that circulate in the blood and enter infected tissues, and macrophages that remain inside tissues. They both destroy bacteria and viruses by phagocytosis.

4 Antigens are ‘foreign’ (non-self) macromolecules that stimulate the immune system.

5 Lymphocytes also originate in bone marrow. There are two types: B-lymphocytes (B cells) and T-lymphocytes (T cells). As they mature, B cells and T cells gain glycoprotein receptors that are specific to each cell. Each cell divides to form a small clone of cells that spreads throughout the body in the blood and in the lymphoid tissue (e.g. lymph nodes and spleen).

6 B cells mature in bone marrow. T cells mature in the thymus gland. During maturation, many T cells are destroyed, as they express receptors that interact with self-antigens. If left to circulate in the body, they would destroy cells and tissues. The T cells that are not destroyed recognise non-self antigens, such as those on the surfaces of pathogens.

7 During an immune response, those B and T cells that have receptors specific to the antigen are
activated.

 8 When B cells are activated they form plasma cells which secrete antibodies.

9 T cells do not secrete antibodies; their surface receptors are similar to antibodies and identify
antigens. They develop into either T helper cells or killer T cells (cytotoxic T cells). T helper cells
secrete cytokines that control the immune system, activating B cells and killer T cells, which kill
infected host cells.

10 During an immune response, memory cells are formed which retain the ability to divide rapidly and develop into active B or T cells on a second exposure to the same antigen (immunological memory). The diagram on the next page summarises the actions of B cells and T cells during an immune response.

11 Antibodies are globular glycoproteins. They all have one or more pairs of identical heavy polypeptides and of identical light polypeptides. Each type of antibody interacts with one antigen via the specific shape of its variable region. Each molecule of the simplest antibody (IgG) can bind to two antigen molecules. Larger antibodies (IgM and IgA) have more than two antigen-binding sites.

12 Antibodies agglutinate bacteria, prevent viruses infecting cells, coat bacteria and viruses to aid
phagocytosis, act with plasma proteins to burst bacteria, and neutralise toxins.

13 Active immunity is the production of antibodies and active T cells during a primary immune response to an antigen acquired either naturally by infection or artifi cially by vaccination. This gives permanent immunity.

14 Passive immunity is the introduction of antibodies either naturally across the placenta or in breast milk, or artifi cially by injection. This gives temporary immunity.

15 Vaccination confers artificial active immunity by introducing a small quantity of an antigen by
injection or by mouth. This may be a whole living organism, a dead one, a harmless version of a toxin (toxoid) or a preparation of antigens.

1. Multiple-choice test

1  What are produced from a B-lymphocyte?

2  What is the function of plasma cells during an immune response?

      A   to become memory cells
      B   to ingest invading bacteria
      C   to kill cells infected with viruses
      D   to secrete antibodies

3  After an immune response, memory cells remain in the blood for a long time.
    What is the function of a memory cell?

     A  to divide to make plasma cells
     B  to ingest invading bacteria
     C  to kill cells infected with viruses
     D  to secrete antibodies

4  Which of the following describes an antigen?

     A  binds to a molecule that can stimulate an immune response
     B  binds with a non-self protein
     C  does not stimulate an immune response inside the body that produced it
     D  stimulates an immune response when recognised as non-self

5  Newborn babies acquire immunity from their mothers.
    Why is this immunity only temporary?

    A  no memory cells are produced by the baby
    B  not enough antibodies are produced
    C the antibodies act only in the mother
    D the immunity is not inherited

6  The statements describe molecular structure.
      1 an insoluble fibrous glycoprotein
      2 has quaternary structure held together by disulfide bonds
      3 has two identical binding sites
      4 made up of two longer and two shorter chains

Which statements describe an antibody molecule?
         A   1, 2, 3 and 4
         B   1, 2 and 3 only
         C   2, 3 and 4 only
         D   3 and 4 only

7  An individual’s immunity may result from:
     1  having memory cells after an infection
     2  having memory cells after being injected with dead bacteria
    3  being injected with antibodies
    4  receiving antibodies from breast milk.

Which row shows an example of the different types of immunity?

8  The drawing shows some blood cells.

9  The statements are about the role of a phagocyte.
     1  bacteria in phagocytic vacuole
     2  hydrolysis of bacteria
     3  phagocyte attracted to bacteria
     4  bacteria taken into phagocyte by endocytosis
     5  fusion of lysosomes with phagocytic vacuole

Which of the following shows the sequence of events after antibodies have become attached to a pathogenic bacterium?
     A   1 → 5 → 2 → 3 → 4
     B   3 → 4 → 1 → 5 → 2
     C  4 → 1 → 3 → 5 → 2
     D   5 → 2 → 3 → 4 → 1

10 The statements describe lymphocytes.
     1  Each B-lymphocyte can make only one type of antibody.
     2  Both B-lymphocytes and T-lymphocytes circulate in the blood and lymph.
     3  Some T-lymphocytes stimulate B-lymphocytes to divide.
     4  B-lymphocytes can develop into plasma cells and secrete antibodies.
     5  Some T-lymphocytes kill any of the body’s cells that are infected with pathogens.

Which statements are correct?
     A  1, 2, 3, 4 and 5
     B  1, 2, 3 and 4 only
     C  2, 3 and 4 only
     D  3, 4 and 5 only

Answers to Multiple choice test

1. D

2. D

3. A

4. D

5. A

6. C

7. B

8. B

9. B
10. A

2. End-of-chapter questions

1  A student  made drawings of four blood cells shown in  the figure.

The  correct   identification   of the  cells is:

2    The  following   occur  during   the  response   to  infection.

1  bacteria   attach   to cell surface  membrane     of phagocyte

2    movement    of phagocyte    to  site  of infection    by bacteria

3    formation    of a phagocytic  vacuole

4    fusion   of lysosomes   to  the  phagocytic  vacuole

5    infolding    of cell surface  membrane

6    release  of enzymes   into  the  phagocytic  vacuole

In  which   order  do  these  events  occur?

A   1,2,3,4,6,5

B   1,2,3,5,4,6

C  2, 1, 3, 6, 5,4

D  2, 1,5,3,4,6

3    Which  of the following explains why antibody molecules have quaternary structure?

A   antibodies    have  a variable   region

B   antibodies    have  complex   3D  shapes

C   antibodies    have  four  polypeptides

D   antibodies    have  more  than   one  polypeptide

4    Which   type  of immunity    is provided    by vaccination?

A   artificial   active

B   artificial   passive

C   natural   active

D   natural   passive

5    Tetanus   is a bacterial   disease  that  may  be acquired   during   accidents   in which   a wound   is exposed   to the  soil.

B-lymphocytes    originate    from  stem  cells,  mature   and  circulate   around   the  body.  Following   infection   by tetanus bacteria,   some  B-lymphocytes   will  become   activated   as shown   in  the  figure.

a    With   reference   to  the  figure,   name:

i  the  place  where   the  stem  cells divide   to form B-lymphocytes   [1]

ii the  type  of division   that  occurs   at E                   [1] 

iii the  activated   B-Iymphocyte,   F                              [1] 

iv   the  molecule   G.                                                     [1]

b    Use  the  information     in  the  figure  to  explain the differences between the following pairs of terms:

i  antigen   and  antibody                                                 [3]

ii self and  non-self                                                          [2]

c      Explain   how  cell  H  is responsible    for  long-term immunity to tetanus.               [3]

[Total:  12]

6  Phagocytes  and  lymphocytes   are both  present   in  samples   of blood.

a  Describe how the structure of a phagocyte differs from the structure of a lymphocytes.                                                                                 [3] 

T-lymphocytes  are involved   in immune    responses   to pathogens    that  invade   the  body.  Immune    responses   involve  the following:

•  antigen  presentation

•  clonal selection

•  clonal  expansion

Certain  groups   of T-lymphocytes    are activated   when   the  body  is infected   with   the  measles  virus.

b   Using  the  information     above,  describe   what  happens   to T-lymphocytes   during   an  immune    response   to  measles.    [6]

c   State how  the  response   of B-lymphocytes   during   an  immune    response   is different   to  the  response of T-lymphocytes.      [2]                                                                                                     

[Total:  11]

7    Measles   is a common    viral  infection.  Babies  gain  passive  immunity to  measles. 

    a    Explain:

i  the  term  passive   immunity  [1]

ii how  babies  gain  passive  immunity. [2] 

 A vaccine   for  measles  has  been  available   since  the  1960s.  Global   vaccination    programmes     include   providing vaccination    for  measles,   but  it is important     that  the  vaccine   is not  given  to babies  too  early.

b    Explain   why:

i  the  vaccine   for  measles  should   not  be given  too  early    [3] 

ii    measles  has  not  been  eradicated,    even  though   a vaccine   has  existed  since  the  1960s.  [3] 

c      Smallpox   was  an  infectious    disease  that  was finally  eradicated    in  the  late  1970s.  Explain   how  vaccination was  used  in  the  eradication    of smallpox. [6]

 [Total: 15]

8    The  figure  is a diagram   of an  antibody    molecule.

a    Describe   briefly  how  antibody    molecules   are produced    and  secreted. [4]

b    Name:

i  the  regions  X and  Y [2]

ii the  bond   labelled   z.  [1] 

c    Explain   how  the  structure    of an  antibody    is related   to its function.[4]

[Total: 11]

3. End-of-chapter answers

 1   C
 2   D
 3   D
 4   A

Exam-style questions


5 a i bone marrow; [1]
      ii mitosis; [1]
     iii plasma cell; [1]
     iv antibody; [1] 

  b i antigen refers to any substance that stimulates the production of antibodies; antibodies are                  proteins produced by, plasma cells/(activated) B-lymphocytes; each antibody is specific to an antigen; [3]
    ii self refers to antigen(s) within a person’s body (e.g. those of the ABO blood group system which they have); all the antigens that the immune system does not recognise as foreign; [max. 1]

        non-self refers to antigen(s) that are not in a person’s body (e.g. those of, pathogens/the ABO system, that they do not have); all the antigens that the immune system recognises as foreign;              [max. 1]

c   memory cell;
     remains in circulation/lymph system/body;
     is specific to an antigen on tetanus bacteria;
     responds quickly to another infection by (same strain of) pathogen;
    as there are a large number/is a large clone;
    during (secondary/subsequent) immune response;
    differentiate into plasma cells;
      to give large number of antibody molecules in short space of time; [max. 3]

  [Total: 12]

6 a phagocyte has lobed nucleus;
      lysosomes;
      larger quantity of cytoplasm; [3]
      Reject statement that phagocyte is larger, as question asks for diff erences in structure, not size.

 b presentation of antigen(s) by macrophages/other APCs;
     some T-lymphocytes have receptors complementary to antigen;
     these are selected;
     divide by mitosis;
     helper T cells secrete cytokines;
     to activate B-lymphocytes;
     to secrete antibodies;
     killer T cells search for cells infected by, parasite/ pathogen; 
    destroy host cell (and pathogen);
    prevent reproduction of pathogen; [max. 6]

 c B-lymphocytes can be activated by presence of, antigen/pathogen alone;
    without involvement of macrophages;
    B-lymphocytes diff erentiate into plasma cells;
    secrete antibodies (T cells do not secrete antibodies); [max. 2]

 [Total: 11]

7 a i immunity is gained by the transfer of antibodies from another source;
     no immune response within the body; 
     antigen(s)/pathogen(s),
    have not entered the body; [max. 2]

    ii natural passive immunity: antibodies cross the placenta;
      in breast milk/colostrum; [max. 1]

  b i baby has passive immunity;
      antibodies against measles antigens (from mother) will interact with measles viruses/ antigens in         vaccine;
     so prevent an immune response;
     therefore no memory cells will be formed; [max. 3]

   ii difficulty with timing first vaccination; 
     many children are not vaccinated at appropriate time;
     measles is highly infectious;
     vaccination programmes concentrated on other infectious diseases which have more severe effect,      such as smallpox and polio; [max. 3]

   c herd/mass vaccination/immunity;
      prevented spread through population;
     surveillance for infected people;
    very easy to identify infected people/no symptomless carriers;
    contact tracing to fi nd people who may have become infected; ring vaccination/vaccination of all        people in surrounding area;
    prevented spread from isolated infected people;
    one dose of the vaccine was enough to give lifelong immunity/no boosters required;
   vaccine contained ‘live’ virus;
    smallpox virus was stable/did not mutate;
    no antigenic variation;
   same vaccine was used for whole programme/did not need to be changed; heat-stable/freeze-dried,       vaccine;
    suitable for hot countries/isolated areas/rural areas;
    bifurcated/steel, needle made vaccinating easy;
    did not need to be done by health professionals; [max. 6]

   [Total: 15]

8 a transcription (of DNA);
      translation (of RNA);
      assembly of amino acids to make four polypeptides;
      assembly of polypeptides to make antibody molecule; 
      packaged in Golgi body into vesicles;
     exocytosis; [max. 4]

   b i X = variable region/antigen-binding site; Y = constant region; [2]
      ii disulfide; [1]

   c variable region(s) are antigen-binding sites;
      variable regions, are specific/complementary, to antigen;
       variable region has diff erent amino acid sequences for diff erent antigens;
      20 different amino acids can be arranged to form diff erent shapes;
      disulfide bonds hold polypeptides together;
      hinge region allows flexibility in binding to antigen;
      constant region for binding to receptors on phagocytes; [max. 4]

 [Total: 11]