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Antibody Concentration – Primary and Secondary Response

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Primary Response Infection (Ag) Lag phase 3 Antibodies produced 4 Antibody level rises to combat infection 5 Ag dealt with 6 Ab level declines – short lived. Antibody Concentration – Primary and Secondary Response. Secondary Response

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Primary Response
  • Infection (Ag)
  • Lag phase
  • 3 Antibodies produced
  • 4 Antibody level rises
  • to combat infection
  • 5 Ag dealt with
  • 6 Ab level declines – short lived

Antibody Concentration – Primary and Secondary Response

Secondary Response

After the primary response, Ab’s do not stay in blood – the level declines

If the body is infected by the same Ag a second time Ab’s must be made again

Re-infection causes much more rapid and a stronger immune response – concentration of Ab’s rises sooner- reaches a higher concentration – more plasma cells than in 1o response – more cells to respond to Ag; less time to produce same number of plasma cells –hence, a greater [Ab] compared to 1o response; increased affinity of Ab for Ag.

This is due to the presence of memory cells (made during the primary response) – no need for antigen presentation and clonal selection

Long-lived; basis of vaccination

Lag phase – antigen presentation (by macrophage /phagocyte); clonal selection (T + B lymphocytes); production of cytokines; B cell activation; clonal expansion; formation of plasma cells; protein (antibody synthesis

B cells

Humoral response

Has Ag receptors – carries Ab (receptor) on surface

Complimentary to only one Ag

Clonal selection

Each B cell – molecules of single type of Ab on outer surface of plasma membrane

Selection and activation of appropriate B lymphocyte / B cell and T lymphocyte by APC’s (macrophages)

Clonal expansion

T cells divide by mitosis to form a clone (clonal expansion) - and secrete signal molecules termed cytokines (lymphokines) which stimulate the selected B cells to divide by mitosis to form a clone (clonal expansion)

B cells specialise / differentiate to form larger Ab secreting plasma cells

Ab’s are specific / complementary to Agnitiating the response

B cells and T cells also differentiate into memory cells – long lived / remain in circulation – provide immunological memory to provide a secondary response on subsequent exposure to the same Ag; 2o response is more rapid and stronger; produces larger amount of Ab

Phagocyte (has enzymes)

Partial digestion of Ag

Ag still whole

Antigen presenting cell

(antigen presentation)

Clonal selection – receptors

On T cell membrane – complementary to Ag

Clonal expansion (T cells) – divide by mitosis

Clonal expansion (B cells)

Remain in body to produce a rapid and stronger 2o response on exposure to the same Ag – divide to make Ab producing plasma cells –



Ag selects B

Cell with right

Shape of recepror



Divide by mitosis

To form a clone of

Plasma cells and a

Clone of memory


B cell


Origin – bone marrow + foetal liver cells

Development – bone marrow + foetal liver

Low nucleus to cytoplasm ratio – nucleus larger relative to cytoplasm

Plasma cell


Derived from B cell

Large nucleus to cytoplasm ratio – nucleus smaller relative to cytoplasm

Develops extensive RER - increase in protein (antibody) synthesis; transport

More ribosomes; more mitochondria (ATP for synthesis and secretion)

More Golgi apparatus – for secretory vesicles;adding carbohydrate (to make glycoprteins)

More space for organelles

Ab (protein) made by ribosomes (RER); ATP required (mitochondria)

Ag binding site (Fab)

Variable region – varies from one Ab to another – due to variable amino acid sequence (primary structure) – different 3o and 4o structures; different shapes antigen binding sites - ensures specificity for a particular Ag

Specific shape – complementary

to Ag – “Lock and Key”

Bind with a specific Ag

Produced by B lymphocytes

Large globular proteins – Y shaped

4 polypeptide chains (held together by S-S (disulphide) bonds

An antibody molecule

Hinge region – allows spatial

flexibility to branches of the Y shaped variable region of the Ab

Allows for binding to more than one Ag

Constant region (Fc)

Related to class of Ab

Enables Ab to bind to receptors and attach to cells – e.g. Phagocytes / mast cells

Neutralisation and agglutination

Mode of Action of Antibodies

  • also promotes phagocytosis by
  • the constant region attracting
  • phagocytes

Immobilise pathogens (bind with flagellum)

Destroy bacterial cell wall (lysis)

Stop spread

Cell Signalling in Immune Response

Communication between cells

T cells

Ag presentation by phagocytes

Receptors on T cell surface complementary to Ag – specificity

Clonal selection – Ag selects only those T cells with complementary receptors and activates them

Clonal expansion - activated T cells divide by mitosis into a clone

Th cells release cytokines (signalling chemicals)

Stimulate B cells to divide by mitosis to form a clone of B lymphocytes (low nucleus to cytoplasm ratio)

These differentiate into larger plasma cells (large nucleus to cytoplasm ratio)

Secrete antibodies

Tcyt – kill cells with intracellular parasites

Tk – kill abnormal cells with markers for cancer

Tsup – regulate immune response

Memory T cells – able to differentiate rapidly into Th, Tcyt, Tk cells on stimulation with Ag


Flow chart to represent

the immune response