Test 126 (Immunology)

1.  Immunoglobulins:

a. contain antigen-binding sites derived from both the 
    heavy and light chains.

b. contains Fc regions derived solely from the heavy 
    chains

c. are chiefly of Ig E class at mucosal surfaces

d. are produced by both T and B lymphocytes

e. of Ig E class give protection against parasitic worms.

Answera.T   b.T   c.F   d.F   e.T
The antigen binding sites are composed of the 
hypervariable regions of both heavy and light chains 
which together make the complementarity determining 
region (CDR) of the antibody. The Fc is made up of 2 H chains, linked by disulphide bonds. IgA (actually 
IgA2), constitutes the major antibody in secretions, and 
bathes mucosal surfaces. IgA1 is mainly confined to the 
serum. Only B lymphocytes are capable of synthesising antibodies, as also are plasma cells. Worm antigens are constantly being released, and attach to specific IgE antibody that in turn has attached to mast cells by their
receptor for IgE. Interactions with IgE results in mediator 
release by the mast cell, one effect of which is contraction of smooth muscle which is intended to expel the parasite from the gut. 

2. Opsonization:

a. is mediated by complement components.

b. enhances phagocytosis

c. involves mainly the Fc protion of the immunoglobulins

d. involves fibronectin

e. is not restricted by the MHC (Major Histocompatiblity 
    Complex

Answera.T   b.T   c.T   d.T   e.T
Opsonins include C3b, for which phagocytic cells possess a receptor. Phagocytosis, though possible
in a saline medium,is enhanced in the presence of 
fresh serum. This is due to substances called 
opsonins.Immunoglobulins, especially IgG1and 
IgG3, for the Fc portion of which phagocytic cells possess receptors, are potent opsonizing agents. 
Fibronectin acts as a glue for neutrophil-target interaction and is a glycoprotein. Opsonization is not MHC-restricted. 

3. IgG:

a. has a molecular weight of 50,000 kd.

b. is monvalent

c. comprises the majority of circulating antibody in serum

d. differs from other isotypes in being ablt to cross the 
    placental barrier

e. is the major antibody produced during the primary 
   response

Answera.F  b.F   c.T   d.T  e.F
Each light chain has a MW of 25,000 and each H chain a MW of 50,000. Therefore, since the whole 
molecule consists of 2 L and 2 H chains, the MW is 
150,000 kd. It exists as a monomer with 2 Fab 
portions, each of which can interact with an antigenic 
determinant. Therefore it is divalent. Normal range 8-19 g/l. Next is IgA, 1-5 g/l, followed by IgM 0.5- 2 g/l. It is in fact the only antibody capable of crossing the 
placental barrier, which it does through gaining attachment via its Fc portion. It is the major antibody produced in the secondary immune response. IgM is the major antibody produced during the primary response.

4. For antigen presentation to CD4+ T lymphocytes:

a. antigen is processed via the endogenous pathway

b. specialised antigen presenting cells are required for the 
    induction of the T cell immune response

c. Short antigen-derived peptides associate with MHC 
    class II molecules in the endoplasmic reticulum

d. exogenously added synthetic peptide corresponding to 
    the antigenic epitope can substitute for processed 
    antigen for T cell recognition. 

e. the antigen must be synthesised within the antigen 
    presenting cell in order to be processed and presented 
    by MHC

Answera.F   b.T   c.F   d.T   e.F
Antigen processed via the exogenous pathway for 
presentation by MHC class II molecules. Specialised 
antigen presenting cells (dendritic cells, macrophages, B cells) are required for the induction of the T cell immune response. Peptides associate with MHC 
class I molecules in the endoplasmic reticulum. 
Association of MHC class II and peptide occurs in 
an endosomal compartment of the cell. Exogenously 
added synthetic peptide corresponding to the antigenic epitope can bind to MHC at the cell surface and 
substitute for processed antigen in T cell recognition. 
Peptides presented by MHC class II molecules to CD4+ T cells are usually derived from exogenous antigens. 

5. In the thymus:

a. the majority of cortical thymocytes express either CD4 
    or CD8.

b. CD4/CD8 double positive cells are eliminated by a 
    process of negative selection.

c. a proportion of alpha/beta+ thymocytes undergo 
    isotype switching to produce gamma/delta+ T cells.

d. thymocytes whose TcR bind with high affinity to self  
    Ag/MHC complexes are clonally detected.

e. mature thymocytes express surface IgM and IgD

Answera.F   b.F   c.F   d.T   e.F
Cortical thymocytes are immature forms, and either 
do not express CD4 or CD8 (double negative cells)
or express both CD4 and CD8 (double positive cells).
As the cells mature, they pass to the thymic medulla, 
where they lose expression of either CD4 or CD8, to 
become single positive cells. Negative selection occurs 
at the stage when thymocytes express both CD4 and CD8, but co-expression of these markers does not 
mediate negative selection. Negative selection occurs 
when a thymocyte expresses a TcR with high affinity for self antigen:MHC complexes in the thymic 
micro-environment. Once a thymocyte has successfully rearranged and expressed an alpha/beta or 
gamma/delta TcR it is committed to that lineage. 
Thymocytes whose TcR bind with high affinity to self 
Ag/MHC complexes are clonally deleted by a process of negative selection. B cells express IgM and IgD but not T cells.

 
 

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