Untitled Quiz

Humoral Immunity

• Humoral immunity is a branch of the adaptive immune system
• Involves antibodies produced by B cells
• Antibodies circulate in bodily fluids like blood and lymph, targeting and neutralizing pathogens.

B Cell Recap

• B cells develop in bone marrow (also in birds, rabbits, and pigs)
• Three stages of development:
  • Maturation: Immunocompetent B cells develop in bone marrow
  • Activation: Contact with antigen triggers activation
  • Differentiation: B cells differentiate into plasma cells (for antibody production) and memory B cells
• Sites of Development: Bone marrow, Bursa of Fabricus (birds), Peyer's Patches (other mammals).
• B cells activated by external antigens in lymph node and by blood-borne antigens in the spleen.
• Plasma cells secrete immunoglobulins specific to antigens.

B Cell Receptors

• B cells have 200,000-500,000 identical BCRs per cell
• BCRs are composed of two heavy chains and two light chains
• BCRs include soluble forms (antibodies), antibodies bind to specific antigens
• BCRs include Ig-a and Ig-b (CD79a & CD79b) glycoprotein heterodimers that help transduce activation signals.
• Longer tails called ITAMs (immunoreceptor tyrosine-based activation motifs) extend into the cell to signal.

B Cell Activation

• Driven by the presence of antigen (multivalent antigen - repeated epitopes)
• Naive B cells that don't encounter antigen die within a few weeks
• T-independent antigens can directly activate B cells without helper T cells (weak response).
• T-independent antigens include mitogenic antigens and cross-linking antigens
• T-dependent antigens require helper T-cell activation for a stronger response.
• A short-lived plasma cell response, no memory.
• The B cells that act independently are called Marginal Zone B cells (secondary lymphoid organs) or B-1 cells (mucosal tissues & peritoneal cavity)

B Cell Activation (T-Cell Dependent)

• Has helper T-cell support (occurs in secondary lymphoid organs)
• One activated B cell can activate up to 5000 antibody-secreting cells.
• Each antibody-secreting cell can produce 2000 antibodies per second.
• Isotype switching: Changes from IgM to more durable, high-affinity IgG, IgA, or IgE
• Affinity maturation: B cells producing high-affinity antibodies dominate the response
• Long-lived plasma cells & memory B cells. Mediated by Follicular B cells (B-2 cells)

Humoral Response (Primary)

• Characterized by a lag phase caused by:
  • Clonal selection: selecting the B-cell with the highest antigen specificity
  • Clonal expansion: creating many identical B cells
  • Differentiation: producing plasma and memory cells to produce antibodies
• Mostly IgM and IgG in primary reaction.
• Characterized by low antibody level and IgM dominance.

Humoral Response (Secondary)

• Memory B cells stimulated by subsequent antigen contact.
• Dependent on the existence of memory B and helper T cells
• Stronger, faster, and lasts longer
• Higher affinity antibodies; instant response.
• Higher levels of IgG than IgM.

Co-stimulation of B cells

• B cells require a two-factor signal to activate from helper T cells.
  • Signal 1: helper TCR and MHC class 2 from B cell.
  • Signal 2: CD40-CD40L induces B7 and CD28.
• Cytokines are released

Humoral Immunity (General)

• Antigen arrives in secondary lymphoid organ.
• B cell recognition of antigen triggers initial response.
• BCR binds to antigen initiating intracellular signaling
• B cells then present antigen in MHC class II molecules to helper T cells
• Helper T cells activate B-cells, further differentiating and proliferating to become plasma cells.
• Short-lived plasma cells produce antibodies immediately
• Activated B and T cells migrate into follicles forming germinal centers where differentiation continues.
• A germinal center is composed of the dark and light zones.
• Somatic mutation, isotype switching, and affinity maturation in the light zone

Germinal Centers

• Within follicles of secondary lymphoid tissues.
• Basal dark zone and light zone are present.
• Proliferating B cells in the dark zone.
• Follicular dendritic cells and follicular helper T cells in the light zone to cause somatic mutation, affinity maturation, and isotype switching.
• High affinity antibody-secreting cells and memory cells exit the germinal center

Isotype Switching

• Involve genetic alterations in the immunoglobulin heavy-chain loci

Affinity Maturation

• Occurs by somatic hypermutation through point mutations, deletions, and insertion of Ig genes.
• Selection of high-affinity B-cells to create better binders and neutralizers via high-affinity antibody production.

Clonal Selection

• B cells binding antigens in germinal centers survive.
• B cells with low affinity to foreign antigens are eliminated via clonal deletion.

Plasma Cell Differentiation

• Short-lived plasma cells result from T-independent responses during extra-follicular response.
• Long-lived plasma cells are generated by T-dependent responses in germinal centers. -These produce antibodies for decades after antigen removal.

Memory B Cells

• Generated during germinal center reaction, or in T-independent responses.
• Mostly during T-dependent responses.
• Make rapid response to subsequent antigen encounters
• Long-lived due to anti-apoptotic BCL-2.
• Remain in lymphoid organs, some circulate.

Antibodies (General)

• Antibody functions include neutralization, opsonization, ADCC, and complement activation.

Antibody Structure

• Composed of 4 polypeptide chains: 2 heavy chains and 2 light chains.
• Determined by isotypes (alpha, gamma, delta, epsilon, and mu), and subtypes that differ slightly.
• Connected by disulfide bonds and non-covalently hydrogen and hydrophobic bonds.
• Variable and constant regions.
• Fragment antigen-binding region (Fab) - antigen binding
• Fragment crystallizable region (Fc) - antibody interactions

Antibody Functions

• Neutralization, opsonization, ADCC (antibody-dependent cellular cytotoxicity), and complement activation.

Immunoglobulin Classes

• Each class has a unique amino acid sequence in the constant region of the heavy and light chains
• IgG, IgM, IgA, IgE, and IgD

IgM

• First immunoglobulin synthesized in newborns.
• Most effective complement activator.
• Structurally, its five subunits are joined by joining chains.

IgA

• Dominates in body secretions (milk, tears, mucus)
• Protects from pathogens that enter vulnerable mucosal portals.

IgE

• Small amounts in serum; allergic reactions, and parasite immunity.

IgD

• Found on the surface of B cells; not much biological function.

Anitgenic Determinants on Ig

• Isotypic: Species-specific constant regions of heavy and light chains
• Allotypic: Differences in amino acids within the same Ig class within a species
• Idiotypic: Differences in the variable regions; unique to individual antigens

Methods of Determining Antibody Structure

• Digested Ig with enzyme papain into Fab and Fc regions
• Cleaved above the disulfide bond.
• Digested Ig with enzyme Pepsin into an Fab complex, and a partial Fc, and cleaved below the disulfide bond.

Other notes:

• B and T cell differentiation, maturation, and activation occur in different areas of lymphoid organs (liver, spleen, lymph nodes).
• Some processes that occur in a T-dependent B cell activation include antibody secretion, isotype switching, affinity maturation, and memory cell formation.
• B cells require a two-factor signal for activation.
• Multiple factors act on B cells prior to their differentiation into plasma cells.