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11.1.310-11 · Infectious Disease and Immunity

Primary Immune Response — Cambridge AS & A Level Biology 9700 Revision Notes & Topical Questions

5 min read 10 parts Lesson 13 of 21 1 quiz

📚 Cambridge AS & A Level Biology 9700 Hub / Topical Revision
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11.1.3 Primary Immune Response — CAMBRIDGE AS & A LEVEL BIOLOGY 9700 — REVISION NOTES notes: Describe how B cells and T cells respond when an antigen is met for the first time.Definition — the primary immune response occurs when the body meets an antigen for the first time.Delay …

  • Two branches
  • Coordination
  • Starting point
  • Importance
  • Antibody actions
  • Comparison

Part 1 of 8Overview

Definition
  • Definition the primary immune response occurs when the body meets an antigen for the first time.
Delay
  • Delay few lymphocytes recognise it, so it takes about 5 to 10 days to build up effector cells, and the patient is ill meanwhile.
Two branches
  • Two branches the humoral response and the cell-mediated response (T cells destroy infected cells).
    • (B cells make antibodies)
Coordination
  • Coordination helper T cells coordinate both.
Starting point
  • Starting point a non-specific response comes first, then the specific response.

Part 2 of 8The cell-mediated response

1
A macrophage engulfs the pathogen and presents its antigen.
2
A helper T cell with a complementary receptor binds the presented antigen and is activated.
3
The helper T cell divides by mitosis to form a clone and releases cytokines.
4
Cytokines stimulate cytotoxic (killer) T cells with matching receptors, B cells and phagocytes.
5
Cytotoxic T cells bind to infected body cells showing the antigen.
6
They release perforin, which makes pores in the membrane, and other enzymes that trigger cell death (apoptosis).
7Importance
  • Importance it is vital against viruses, which live inside cells where antibodies cannot reach, and against cancer cells and transplanted tissue.
8HIV
  • HIV it destroys helper T cells, so the whole immune system fails.

Part 3 of 8The humoral response

A B cell with a receptor complementary to the antigen binds it, takes it in and displays a fragment.
A helper T cell recognising the same antigen binds to the B cell and releases cytokines, activating it (clonal selection).
The activated B cell divides by mitosis (clonal expansion).
Most cells of the clone become plasma cells.
Plasma cells have extensive rough endoplasmic reticulum and Golgi apparatus and secrete large quantities of specific antibodies into plasma and lymph, at up to 2000 per second.
Some cells become memory B cells.
Antibody actions
  • Antibody actions they neutralise toxins, agglutinate pathogens and help phagocytes engulf them (opsonisation).

Part 4 of 8B cell and T cell details

B cells
  • B cells each mature B cell has antibody receptors on its surface, specific to one antigen.
  • They mature in the bone marrow and concentrate in lymph nodes and spleen.
T cells
  • T cells they mature in the thymus and carry T cell receptors specific to one antigen.
  • T helper cells release cytokines that stimulate B cells and increase phagocytosis by macrophages.
  • T killer cells attach to antigens on infected cells and secrete toxic substances.

Part 5 of 8Cells in the primary response

CellRole
MacrophageEngulfs and presents antigen
Helper T cellCoordinates by cytokines
Cytotoxic T cellKills infected cells
B cellSelected and cloned
Plasma cellSecretes antibodies
Memory cellRemains for future exposure

Part 6 of 8Antibody graph in the primary response

1Lag phase
  • Lag phase several days with no detectable antibody.
2Rise
  • Rise antibody concentration climbs to a modest peak.
3Fall
  • Fall it declines as the pathogen is cleared.
4Comparison
  • Comparison the secondary response is faster, larger and longer-lasting.

Part 7 of 8An example: influenza

Antigen capture
  • Antigen capture macrophages engulf virus and present fragments, and a matching helper T cell binds, divides and releases cytokines.
Antibodies
  • Antibodies activated B cells become plasma cells making antibodies to viral surface proteins, which neutralise virions and tag them for phagocytosis.
Infected cells
  • Infected cells cytotoxic T cells recognise viral antigens on epithelial cells and destroy them with perforin before new virus is assembled.
Symptoms
  • Symptoms fever, aches and tiredness are partly caused by cytokines released in the response.
Harmful responses
  • Harmful responses the immune response can be harmful in allergy and autoimmune disease.

Part 8 of 8More from the specification

  • Neutrophil
    • A phagocytic white blood cell with a multilobed nucleus.

Key terms

TermMeaning
CytokineA chemical signal released by helper T cells.
Cytotoxic T cellA T cell that kills infected body cells.
PerforinA protein that makes pores in target cell membranes.
Humoral responseImmunity based on antibodies made by B cells.
Cell-mediated responseImmunity in which T cells destroy infected cells.
Helper T cellA T cell that coordinates the immune response by releasing cytokines.

Avoid these mistakes

1
Saying antibodies reach viruses inside cells — cytotoxic T cells deal with infected cells.
2
Saying B cells are activated without helper T cells — helper T cells release cytokines to activate them.
3
Saying the primary response is immediate — it takes several days.
4
Naming the wrong cell for an action — name the correct cell for each step.

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