Humoral immunity

Question 1

3 functions of antibodies

Answer 1

Neutralisation, Opsonisation, Complement activation

Question 2

Difference between antibody affinity and avidity

Answer 2

Affinity: strength of interaction between epitope and Ab’s Ag binding site

Avidity: overall strength of Ab:Ag complex. Depends on affinity + number of antigen binding sites on Ab

Question 3

Structure of antibody

Answer 3

Variable region: many many forms

Constant region: 2x
2 light chains (lamda, kappa)
1 heavy chain (micro, delta, gamma, alpha, epsilon)

Question 4

Which is the first antibody made in B cell

Answer 4

IgM (largest antibody, remains in blood to kill pathogens)

Question 5

Which is the main antibody in blood

Answer 5

IgG

Question 6

Which antibody is mainly found in body fluids

Answer 6

IgA

Question 7

Which antibody remains membrane bound, regulates cell’s activation

Answer 7

IgD

Question 8

Which antibody is found in trace amount in bloods, triggers allergic reactions

Answer 8

IgE

Question 9

Describe primary humoral immune response

Answer 9

1. DC move from infection site to lymph node
2. T cell activated by DC
3. Circulating B cells trapped at T cell zone border, activated by Th2 cell
4. B cell moves to primary follicle, forms germinal centre
5. Some B cells differentiate into plasma cells and secrete immunoglobulin, some continue to proliferate with T cells.
6. Clonal expansion and Somatic hypermutation occurs

Question 10

What co-receptor do all B cells express

Answer 10

CD19

Question 11

What is an epitope

Answer 11

Structure recognised by antibody e.g. viral coat proteins

Question 12

Describe recirculation of lymphocytes

Answer 12

• Naive B cell and CD4 T helper cell find each other in lymph node.
• B cell recirculates from blood through lymph node through primary follicle
• If it doesn’t interact with T cell with same specificity: will recontinue recirculation
• If it binds to Ag to T cell zone: proliferates, produces igM, undergoes somatic hypermutation and class switching

Question 13

Which antibody crosses the placenta

Answer 13

IgG

Question 14

Which antibody is most prevalent in breast milk

Answer 14

IgA.

But note that all Ig are secreted in breast milk.

Note: IgA from breast milk stays in baby’s gut, won’t enter blood.

Question 15

Describe thymus dependent B cell activation

Answer 15

1. BCR (membrane bound Ig) binds to cognate antigen, clustering of Ig receptors
2. T cell sends signal to activate B cell
   (T helper cells that recognise peptide:MHC complex activate B cell)

Question 16

Describe thymus independent B cell activation

Answer 16

1. BCR binds to cognate antigen

2. Direct recognition of pathogen by toll-like receptors OR extensive BCR cross linking

Question 17

Describe Thymus independent Type 1 response + antigens it works against

Answer 17

Directly activates B cells via toll-like receptors (no need T cells)

In high Ag conc: non-antigen specific, can stimulate multiple B cells against multiple antigens

In low Ag conc: antigen specifc

ANTIGENS: LPS, bacterial DNA

Question 18

Describe Thymus independent Type 2 response + antigens it works against

Answer 18

Multiple cross linking of BCR. Only activates mature B lymphocytes.

Children don’t have effective type 2 response cos majority of B cell population is immature

ANTIGENS: epitopes of encapsulated bacteria

Question 19

Significance of giving children <2y conjugated vaccines (instead of pure polysaccharide vaccine)

Answer 19

Induces TI-type 1 response.

Polysaccharide would induce TI-type 2 response, not consistently immunogenic in children. Plus high Ag conc leads to non-antigen specific response.

Question 20

Which of the following are NOT contraindications to vaccination:

• acute fever
• live vaccine for pregnant woman
• child being breast fed
• premature child
• previous hx of pertussis, M, M, or R infection

Answer 20

NOT contraindications

• child being breast fed
• premature child (need even more due to lack of maternal antibody)
• previous hx of pertussis, M, M, or R infection