Lecture 23 - Adaptive Immunity

BIOSCI107

What is adaptive immunity?

A highly specific immune response involving B and T lymphocytes. It has memory and improves with repeated exposure.

How is diversity in adaptive immunity generated?

Through random gene rearrangement of V, D, J, and C segments in BCRs and TCRs.

What is immune memory?

After first exposure to an antigen, memory cells remain and respond faster and stronger on re-exposure.

What is the basic structure of an antibody?

Y-shaped protein made of 2 heavy chains and 2 light chains held by disulfide bonds.
Variable region binds antigen
Constant (Fc) region interacts with immune cells

What is an immunoglobulin (Ig) domain?

A ~110 amino acid domain with a beta-barrel structure, forming the structural unit of antibodies.

How many antigen binding sites does an antibody have?

Two (bivalent). IgM, as a pentamer, has 10 total sites.

What is the function of IgM?

It is the first antibody made during an immune response, exists as a pentamer in blood, and is very effective at fixing complement.

What is the function of IgG?

It is the most abundant antibody in the blood, has high affinity, and can cross the placenta to protect the fetus.

What is the function of IgA?

It provides mucosal immunity and is found in secretions like saliva, tears, milk, and mucous (especially in the gut and respiratory tract).

What is the function of IgE?

It plays a key role in allergy and inflammation by activating mast cells and basophils upon allergen exposure.

What is the function of IgD?

It is primarily found as a membrane-bound receptor on naïve B cells and helps initiate B cell activation.

What is the difference between affinity and avidity?

Affinity = Strength of a single antigen-antibody interaction
Avidity = Combined strength of multiple interactions (e.g. IgM has high avidity)

What are CDRs (Complementarity Determining Regions)?

Hypervariable loops in the antibody’s variable domain that contact antigen.
CDR1 & CDR2 come from the V segment
CDR3 spans V(D)J → most diverse

How many CDR loops are in the antigen binding site?

Six total – three from each chain (heavy and light).

What enzymes initiate V(D)J recombination?

RAG1 and RAG2.

What mechanisms enhance antibody diversity?

V(D)J recombination
Imprecise joining of segments (esp. CDR3)
Random heavy/light chain pairing

What is clonal selection?

Antigen binding activates a specific B cell, which then proliferates into a clone of antibody-producing cells.

What is affinity maturation?

Somatic hypermutation improves antibody affinity over time. Better binders are selected.

Where does affinity maturation occur?

In germinal centres of lymph nodes, aided by T follicular helper cells.

What happens in a lymph node follicle?

B cells mutate, divide, and undergo selection → produces plasma cells and memory B cells.

What does vaccination do?

Mimics infection to train memory B and T cells and promote high-affinity IgG production.

How does the immune response change with booster doses?

1st exposure → low IgG
Booster → faster, stronger IgG response due to memory

How does maternal IgG protect newborns?

It crosses the placenta and provides passive immunity during early life.

What is herd immunity?

When ~90% of a population is vaccinated, disease transmission is blocked even for those unvaccinated.