11.1 The Immune System
- Syllabus
- 9700–2028–2029
- Topic
- 11.1
- Level
- AS
Learning objectives
11.1.1Phagocyte action• Phagocyte mode of action (macrophages and neutrophils)11.1.2Antigens• Antigens: self vs non-self antigens11.1.3Primary immune response sequence• Primary immune response sequence:- Macrophages- B-lymphocytes (plasma cells)- T-lymphocytes (T-helper cells, T-killer cells)11.1.4Memory cells and long-term immunity• Memory cells in secondary immune response and long-term immunity
Phagocytes engulf, digest and expose pathogen evidence
Macrophages and neutrophils are phagocytes: they provide an immediate, non-specific defence by recognising, engulfing and digesting pathogens.
- Chemicals released by pathogens or damaged tissue attract the phagocyte by chemotaxis.
- Surface receptors attach to molecules on the pathogen.
- The cell membrane surrounds the pathogen and encloses it in a phagosome.
- Lysosomes fuse with the phagosome; hydrolytic enzymes digest the pathogen.
- Useful products may be absorbed or released, while waste is removed.
| Phagocyte | Additional role |
|---|---|
| Neutrophil | Rapidly enters infected tissue, phagocytoses pathogens and is often short-lived |
| Macrophage | Longer-lived; after digestion, displays pathogen antigen fragments with cell-surface molecules and acts as an antigen-presenting cell |
Antigen presentation follows digestion of the pathogen: the macrophage displays selected antigen fragments, not an intact pathogen it deliberately failed to destroy. Phagocytes do not secrete pathogen-specific antibodies.
Antigens let the immune system distinguish self from non-self
An antigen is a molecule that can be recognised by the immune system and can stimulate a specific immune response when recognised as non-self. Antigens are often proteins or glycoproteins on cell or pathogen surfaces.
| Antigen class | Origin and normal immune consequence |
|---|---|
| Self antigen | Produced by the body's own cells; immune tolerance normally prevents a response against it |
| Non-self antigen | Foreign to the body, for example on a pathogen or transplanted cell; recognition can activate specific lymphocytes |
Different antigens have different molecular shapes. Only an antibody or lymphocyte receptor with a complementary binding site recognises a particular antigen, which gives the later immune response its specificity.
An antigen is not necessarily the whole pathogen and is not defined only as a marker for ordinary cell-to-cell recognition. 'Self' describes origin and immune tolerance; it does not mean the molecule lacks a shape or identity.
The primary immune response selects and expands specific B and T lymphocytes
A primary immune response is the specific response to a newly encountered non-self antigen. Antigen presentation selects lymphocytes with complementary receptors, which then divide and differentiate into cells that act against the pathogen or infected body cells.
- A macrophage presents pathogen antigen fragments; antigens may also be displayed on a pathogen or an infected body cell.
- The B- and T-lymphocytes with receptors complementary to the antigen are selected. This is clonal selection.
- The selected lymphocytes divide by mitosis, producing many clones with the same antigen specificity. This is clonal expansion.
- B-cell clones form plasma cells, which secrete antibodies complementary to the antigen. T-cell clones form T-helper cells that stimulate B-cell division and T-killer cells that attach to infected cells and kill them.
- The antibody response and T-killer action help remove the pathogen or infected cells. The first response is slow because selection, expansion and differentiation take time.
The response is specific because only lymphocytes with complementary receptors are selected. B cells provide the plasma-cell and antibody branch, while T cells provide helper and infected-cell-killing branches. This card stops at the primary effector response; persistent memory and the secondary response are taught separately.
Clonal selection identifies the matching lymphocytes; clonal expansion increases their number. Plasma cells secrete antibodies, whereas T-killer cells kill infected body cells. Do not treat macrophage antigen presentation as antibody production or include memory-cell mechanisms in this primary-response card.
Memory B and T cells make the secondary response faster and stronger
The primary response leaves antigen-specific memory B and T cells in the blood after the infection has been cleared. Their persistence can provide long-term immunity to the same antigen.
- First exposure: the correct lymphocytes are initially scarce, so clonal selection, mitotic expansion and differentiation take time. Plasma cells develop later and begin producing antibodies; symptoms may occur before the response is fully developed.
- Same antigen again: memory cells are already present in larger numbers. They detect the antigen, activate and divide more quickly, producing a faster and stronger secondary response.
- Functional difference: plasma/effector cells act during the immediate response, whereas memory cells persist as a ready population for future exposure. The secondary response produces antibodies more quickly and at a greater concentration, so it may remove the pathogen before symptoms appear.
Primary response → memory B/T cells persist → same antigen is encountered again → matching memory cells activate rapidly → a larger antibody and cellular response removes the pathogen sooner. This memory principle explains how vaccination can provide long-term immunity; vaccine types and vaccination programmes are taught separately.
Memory is specific to the antigen previously encountered, not a general protection against every pathogen. Memory cells are not the same as plasma cells that immediately secrete antibodies, and a faster response does not require invented time, titre or cytokine values.