antibodies, T cell receptors and MHC

Question 1

What are the sources of infection?

Answer 1

Bacteria
Viruses
Fungi
Parasites
- Worms
- Protozoa

Question 2

What are the characteristics of specific/adaptive immunity?

Answer 2

Induced by exposure to a specific infection
Shows high specificity
Exhibits memory

Question 3

What mediates specific immunity and what are its characteristics?

Answer 3

Mediated by lymphocytes (B and T cells)
Clonally distributed receptors
Large repertoire with a low frequency of cells specific for any antigen

Question 4

What happens during clonal selection?

Answer 4

1. Clonal deletion: Removes potentially self-reactive immature lymphocytes
2. A pool of mature naive lymphocytes forms.
3. Proliferation and differentiation occur upon activation to create effector cell clones.

Question 5

What are the two types of lymphocyte antigen receptors?

Answer 5

BCR (B-cell receptor): Membrane form of Ig that binds free antigens. Becomes an antibody upon activation.
TCR (T-cell receptor): Membrane-bound receptor that recognises peptide fragments presented by MHC molecules on APCs.

Question 6

What are the two main functions of antibodies?

Answer 6

1. Activation of complement:
   
   • Opsonization
   • Classical pathway activation and MAC formation
2. Activation of effector cells:
   
   • Cells expressing Fc receptors (FcR)

Question 7

Describe the structure of an antibody.

Answer 7

4 chains: 2 heavy, 2 light
Variable regions: Form two antigen-binding sites
Constant regions: Determine antibody structure and interact with other immune system molecules (e.g., FcR, complement)
L chain types: λ and κ
5 classes: IgM, IgD, IgA, IgG, and IgE

Question 8

How do antibodies interact with antigens?

Answer 8

Antigen-binding site comprises 6 hypervariable loops in the variable region.
Antigen (Ag) binding depends on size/shape and forms non-covalent interactions with the antibody.
Specificity and affinity are determined by the CDR (complementary-determining regions) of the antibody.

Question 9

How does the T-cell receptor (TCR) differ from the BCR?

Answer 9

Does not bind free antigens but recognizes processed peptides.
Peptides must be presented on MHC molecules.
TCR is smaller and membrane-bound only, composed of α and β chains with 4 Ig-like domains.

Question 10

What are the two classes of MHC molecules and their roles?

Answer 10

MHC Class I:

Expressed on all nucleated cells
Presents endogenous peptides
Examples: HLA-A, HLA-B, HLA-C

MHC Class II:

Expressed on APCs (antigen-presenting cells)
Presents exogenous peptides
Examples: HLA-DP, HLA-DQ, HLA-DR

Question 11

Describe the structure of MHC Class I molecules.

Answer 11

Heterodimer:
- α chain (43 kD)
- β2-microglobulin (12 kD)
Peptide-binding groove: Formed by α1 and α2 domains.

Question 12

Describe the structure of MHC Class II molecules.

Answer 12

Heterodimer:
- α (34 kD) and β chains (29 kD)
- Both transmembrane
Peptide-binding site: Formed by polymorphic α1 and β1 domains.

Question 13

What is the interaction between CD4 and MHC Class II?

Answer 13

CD4 binds to the α2 and β2 domains of MHC Class II molecules.