BIOL1019 Chap.9 Adaptive Immunity: T Cells, B Cells and Antibodies
Adaptive Immunity: T Cells, B Cells and Antibodies
Adaptive immunity is the third line of defence: it recognises specific antigens, expands the few lymphocytes that match them and remembers the encounter. This chapter starts with antigen presentation, the pairing of MHC class I on all nucleated cells with CD8 cytotoxic T cells and MHC class II on antigen-presenting cells with CD4 helper T cells.
It follows the cell-mediated arm through recognition, activation, proliferation and destruction, and the antibody-mediated arm through B-cell sensitisation, helper T cell activation, proliferation and differentiation into plasma cells. The five antibody classes and the difference between primary and secondary responses explain why a second exposure is cleared so much faster.
A common integrative question asks how abnormal or cancerous cells are destroyed by both innate and adaptive mechanisms, so the chapter keeps natural killer cells and cytotoxic T cells side by side and shows what each needs in order to recognise its target.
What this chapter covers
- 01
Specificity, versatility, memory and tolerance
- 02
MHC class I and class II
- 03
CD4 and CD8 markers
- 04
Cytotoxic, helper, memory and suppressor T cells
- 05
B cells, plasma cells and antibodies
- 06
Five antibody classes
- 07
Primary and secondary responses
- 08
Helper T cell subsets
Worked example · free
Matching T cells to MHC molecules
- +1Infected liver cells display viral antigen on MHC class I, found on all nucleated cells.
- +1CD8 cytotoxic T cells recognise antigen on MHC I, are activated, proliferate and destroy the infected cells.
- +1The macrophage, an antigen-presenting cell, displays processed antigen on MHC class II.
- +1CD4 helper T cells recognise antigen on MHC II and stimulate both T and B cell responses.
Key terms
- Antigen-presenting cell
- A cell such as a dendritic cell, macrophage or B cell that displays antigen on MHC class II.
- Plasma cell
- A differentiated B cell that secretes antibodies.
- Clonal selection
- The activation of only those lymphocytes whose receptors match a particular antigen.
- Immunoglobulin G
- The largest antibody class, which provides most of the defence and dominates the secondary response.
- Helper T cell
- A CD4 T cell that recognises antigen on MHC class II and stimulates T and B cells.
- Memory cell
- A long-lived T or B cell that remains after a response and reacts quickly to the same antigen later.
Adaptive Immunity: T Cells, B Cells and Antibodies FAQ
How does a cytotoxic T cell kill its target?
After recognising antigen on MHC class I and being activated and proliferating, an active cytotoxic T cell destroys the target in one of three ways: releasing perforin and enzymes that damage its membrane, stimulating the cell to undergo apoptosis, or releasing lymphotoxin that disrupts the cell's metabolism. Memory cytotoxic T cells remain for future exposures.
Why is the secondary antibody response stronger?
During the first exposure, matching B cells proliferate into plasma cells and memory B cells, and the primary response produces low levels of IgM followed by IgG. On a second exposure the memory cells respond at once, so the secondary response is more extensive and prolonged, dominated by enhanced IgG, and the pathogen is usually cleared before illness develops.
What do the helper T cell subsets do?
Activated CD4 helper T cells form subsets with different targets. TH1 cells direct responses against intracellular pathogens such as viruses. TH2 cells support IgE and eosinophil responses to worms and protozoa and drive B cells to become plasma cells. TH17 cells act against extracellular bacteria and fungi, especially in barrier tissues.
What makes adaptive immunity different from innate immunity?
Adaptive immunity responds to specific antigens, while innate defences work the same way against any pathogen. It also shows versatility through clonal selection and expansion, memory through long-lived memory cells, and tolerance of self-antigens. Its lymphocytes are T and B cells, while the innate system's lymphocyte is the natural killer cell, which needs no prior exposure.
Exam move
Draw the MHC and CD pairings and add the T cell that reads each. Write the cytotoxic T cell steps and the B cell steps side by side as two numbered lists, marking where the helper T cell enters. Learn the five antibody classes with one job each, and practise explaining primary and secondary responses in four sentences that name IgM, IgG and memory cells.
Finish by comparing the two killers the course contrasts, natural killer cells and cytotoxic T cells, on activation, recognition and killing method, in a table you can reproduce in two minutes.
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