Humoral Immunity - B cell Activation, Affinity Maturation and Class Switching

Question 1

What do Naive B cells have on their surface? What do they act as?

Where are Follicular B cells found? What do they recognise and produce?

What is the pathway from a Naive B cell takes once activated?

What are the 3 ways in which Antibodies work?

Answer 1

• Membrane-bound IgM and IgD, which act as B cell receptors (BCR)
• Found in Secondary lymphoid organs (Spleen, Lymph nodes) - Recognise protein antigens and produce high-affinity IgG class/switched antibodies
• Naïve B cell → Activated B cell → Effector B cells/Plasma cells and Memory B cells
• Binds to Extracellular microbes and toxins:
  1. Neutralise - prevent binding of antigens to receptors/cells
  2. Opsonisation - ↑Phagocytosis
  3. Complement activation - Opsonisation, Lysis

Question 2

CLASS SWITCHING:
What can B cells do their Ig’s during the immune response?
→ What can it switch IgM into?
→ What can it switch IgG into?

When changing class, what parts of the Ig don’t change?

How do T cells cause B cell class switching?

What changes in the Ig?

What enzyme is involved in this switching process?

Answer 2

• Can produce Abs of different classes but with the SAME SPECIFICITY - Bind to the same antigens
  → IgG, IgA, IgE
  → IgA, IgE
• Light chain and Specificity
• CD40-Ligand on T cell binds to CD40 on activated B cell AND T cell releases Cytokines for class switching to a specific Ab
• DNA rearrangement of its constant regions
• AID (Activation-Induced Cytidine Deaminase)

Question 3

AFFINITY MATURATION:
How do Ig’s change from the early to late immune primary immune response?
→ What is this called?

What occurs in B cells that cause it to produce more mature Ig’s?
→ What are all these mutations known as?

What can these High-affinity antibody-producing B cells differentiate into after the primary immune response?

Answer 3

• Starts off having a lower affinity for antigens, and then has a higher affinity later
  → Affinity Maturation
• Undergo a lot of point mutations in their Ig genes, and these mutations can either increase or decrease its affinity
  → Somatic Hypermutation
• High affinity antibody-producing B cells survive during the immune response and can differentiate into MEMORY B cells for the Secondary immune response