Investigate the innate and adaptive immune systems in mammals, including the response of animal adaptive immunity to infection (third line of defence: humoral and cell-mediated immunity, including the roles of lymphocytes, antibodies and antigens)

What this dot point is asking

NESA wants you to describe the third line of defence in mammals: the adaptive (specific) immune response. You must cover both humoral (B-cell, antibody-mediated) and cell-mediated (T-cell) immunity, name the lymphocyte types, and explain memory cells and the secondary response. This is among the highest-value Module 7 dot points and appears in nearly every extended-response question.

The answer

Adaptive immunity is specific (each lymphocyte recognises one antigen) and has memory (faster, larger response on re-exposure). It develops over 5 to 14 days during a first infection.

Antigens and antigen presentation

An antigen is any molecule (usually a protein or polysaccharide) that triggers an adaptive immune response. Antigen-presenting cells (APCs) including macrophages and dendritic cells engulf pathogens, digest them, and display fragments on MHC class II molecules. Infected cells display intracellular antigen fragments on MHC class I.

Lymphocyte types

All lymphocytes mature into one of three classes.

B lymphocytes (B cells). Mature in the bone marrow. Each B cell has a unique surface antibody (B-cell receptor) that recognises one antigen.

T lymphocytes (T cells). Mature in the thymus. Each T cell has a unique T-cell receptor (TCR) that recognises one antigen displayed on MHC. Two main subtypes:

• Helper T cells (CD4) recognise antigen on MHC class II. They coordinate the response by secreting cytokines.
• Cytotoxic T cells (CD8) recognise antigen on MHC class I. They kill infected cells.

Memory cells. A subset of activated B and T cells become long-lived memory cells.

Humoral immunity (B cells and antibodies)

Targets pathogens in body fluids.

1. A B cell binds its specific antigen.
2. With help from a matching helper T cell (which has recognised the same antigen on MHC class II), the B cell becomes activated.
3. The activated B cell undergoes clonal expansion, producing many identical daughter cells.
4. Most daughter cells become plasma cells, secreting around 2000 antibodies per second specific to that antigen. A fraction become memory B cells.

Antibodies (immunoglobulins). Y-shaped proteins with two antigen-binding sites. Functions:

• Neutralisation. Bind viruses or toxins, blocking their attachment to host cells.
• Agglutination. Clump pathogens together, easing phagocytosis.
• Opsonisation. Mark pathogens for phagocytes that have antibody receptors.
• Complement activation. Trigger the complement cascade, leading to membrane lysis.

The five antibody classes are IgM (first produced), IgG (most abundant, longest-lasting), IgA (mucosal), IgE (allergies and parasites) and IgD (B-cell receptor).

Cell-mediated immunity (T cells)

Targets infected, cancerous or abnormal host cells.

1. An infected cell displays a viral or abnormal peptide on MHC class I.
2. A cytotoxic T cell with a matching TCR binds the MHC-peptide complex.
3. The cytotoxic T cell releases perforin (forms pores in the target's membrane) and granzymes (proteases that trigger apoptosis), destroying the infected cell.

Helper T cells coordinate the wider response. They release cytokines such as interleukin-2 that activate cytotoxic T cells, stimulate B-cell proliferation, and enhance macrophage activity.

Primary and secondary responses

Primary response. First exposure to a pathogen. Takes 5 to 14 days to produce significant antibody. Symptoms may develop while immunity is building.

Secondary response. Re-exposure to the same pathogen. Memory cells recognise the antigen within hours. Antibody production is faster, higher and longer-lasting. Disease is often prevented or reduced to subclinical levels. This is the basis of natural immunity and vaccination.

Examples in context

Example 1. COVID-19 mRNA vaccine and the adaptive response in Australian recipients. When a NSW resident receives a Pfizer COVID-19 vaccine, lipid nanoparticles deliver mRNA encoding the SARS-CoV-2 spike protein into deltoid muscle and lymph node dendritic cells. The cells translate the mRNA and present spike fragments on their MHC class II receptors to naive helper T cells (CD4+). Helper T cells stimulate B cells specific for spike, which proliferate by clonal selection and differentiate into plasma cells secreting anti-spike antibodies. Memory B and T cells persist for years. NSW Health serology data show neutralising antibody titres rise from undetectable at day 0 to peak at day 14 post-second dose, conferring strong protection against severe disease on reinfection.

Example 2. Tetanus boosters and the secondary response. When a Sydney emergency department patient with a dirty wound receives a tetanus booster, the prior primary exposure (childhood DTPa) has already generated memory B cells against tetanus toxin. The booster triggers a secondary response: memory B cells proliferate within 24 to 48 hours and produce high-affinity IgG antibodies that neutralise toxin within days, far faster than the 10 to 14 days a primary response would take. NSW guidelines require a booster every 10 years (or 5 years if the wound is high-risk) because memory cell counts and antibody titres slowly decline. The contrast between primary and secondary kinetics is the basis of all booster schedules.

Try this

Q1. Distinguish between humoral immunity and cell-mediated immunity by reference to the type of lymphocyte and the type of pathogen each targets. [3 marks]

• Cue. Humoral: B cells produce antibodies, target extracellular pathogens. Cell-mediated: T cells (cytotoxic) kill infected cells, target intracellular pathogens.

Q2. A graph shows antibody titre against tetanus toxin: 1 unit per mL at day 0, peaking at 8 units at day 14, declining to 2 units by day 60. After a booster on day 365, titres reach 200 units within 7 days. Calculate the fold increase between primary peak and secondary peak, and explain the underlying mechanism. [3 marks]

• Cue. 200 divided by 8 = 25-fold increase. Memory B cells from the primary response proliferate rapidly and produce higher-affinity IgG more quickly.

Q3. Describe the role of antigen-presenting cells in initiating the adaptive immune response. (a) Name one antigen-presenting cell. (b) Outline how it acquires and presents antigen. (c) Explain how it activates helper T cells. [1+2+3 marks]

• Cue. (a) Dendritic cell (macrophage or B cell also acceptable). (b) Phagocytose pathogen, process antigen, display on MHC class II. (c) Naive helper T cell recognises peptide-MHC II via TCR; with co-stimulation it proliferates and differentiates.