L41: Specific Defence Mechanism

Question 1

Primary vs Secondary lymphoid organs

Answer 1

Primary (Thymus + Bone marrow): Development and Maturation of lymphocyte
Secondary (Lymph node + Spleen): Differentiation of lymphocyte by interacting with antigen and accessory cells

Question 2

Characteristics of specific immune defence

Answer 2

1. Antigen specific
2. Diverse repertoire
3. Immunological memory
4. Self and non-self discrimination

Question 3

Development of B lymphocyte and T lymphocyte

Answer 3

B lymphocyte:
HSC (bone marrow) —> Naive B cell —> plasma cell + memory B cell (lymph node, spleen)

T lymphocyte:
HSC (bm, thymus) —> Naive T cell —> effector cell (CD4+, CD8+) + memory T cell

Question 4

Structure of immunoglobulin

Answer 4

• 2 heavy chains + 2 light chains
• Fab region:
  —> epitope recognition, antigen binding —> Ag-specificity
  —> highly variable (>10^8 ) between antibodies —> numerous different clones of lymphocytes generated —> Diverse repertoire
• Fc region: biological activity, differentiate IgA, IgM, IgG, IgE, IgD
• Bivalent

Question 5

Function of immunoglobulin

Answer 5

1. Neutralisation of microbes and toxins (agglutination + precipitation)
2. Opsonisation (macrophage have receptors for Fc region of IgG)
3. Complement activation (classical pathway, via Ag-Ab complex by IgM)
4. ADCC (NK cells)

• antibody cannot lyse pathogens directly

Question 6

Structure of T cell

Answer 6

• transmembrane T cell receptor: alpha + beta chains
• monovalent
• TCR + peptide antigen on MHC on APC
• Co-receptor CD4 / CD8
• CD8+ T cell: recognise MHC class I on APC —> cytotoxic granules (perforins, granzymes)
• CD4+ T cell: recognise MHC class II on APC —> cytokine (IL, TNF, IFN), cell-cell interaction, activate B cell response
  —> further divided into different subtypes (Th1 (IL2, IFNgamma), Th2(IL4, IL10), Th17, Treg)

Question 7

Phases of specific immune response

Answer 7

1. Recognition
2. Clonal expansion (each lymphocyte has receptors for single antigen specificity —> Ag-specificity: Burnet’s Clonal Selection Theory)
3. Differentiation (into effector cells + memory cells)
4. Elimination of antigens
5. Apoptosis of effector cells
6. Surviving memory cells (much larger secondary response)

Question 8

Secondary response

Answer 8

• Result from immunological memory
• Faster —> shorter activation phase
• Stronger —> efficient effector phase (more and better antibodies, more T cell)
• Memory cells differentiate into effector cells and other memory cells
• Long-lasting protection
• Mild/subclinical infection —> little/no damage

Question 9

Self and non-self discrimination / Tolerance

Answer 9

1. Central tolerance —> Depletion of self-reactive T/B cell during lymphopoiesis
2. Peripheral tolerance —> Deletion, Inactivation, Suppression of self-reactive T/B cell in peripheral tissue