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Antibody Concentration – Primary and Secondary Response

Primary Response

1. Infection (Ag)

2. Lag phase 3 Antibodies produced

4 Antibody level rises to combat infection

5 Ag dealt with

6 Ab level declines – short lived

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

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Primary – establishes immunologicalmemory

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B cells

Humoral responseHas Ag receptors – carries Ab (receptor) on surfaceComplimentary to only one Ag

Clonal selection

Each B cell – molecules of single type of Ab on outer surface of plasma membraneSelection 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 cellsAb’s are specific / complementary to Agnitiating the responseB 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

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

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Clonal selection – receptorsOn 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 –

Phagocyte (has enzymes)Partial digestion of AgAg still whole Antigen presenting cell(antigen presentation)

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ClonalSelection

ClonalExpansion

Divide by mitosisTo form a clone ofPlasma cells and aClone of memory

cells

Ag selects BCell with right

Shape of recepror

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B cellSmallerOrigin – bone marrow + foetal liver cellsDevelopment – bone marrow + foetal liverLow nucleus to cytoplasm ratio – nucleus larger relative to cytoplasm

Plasma cellLargerDerived from B cellLarge nucleus to cytoplasm ratio – nucleus smaller relative to cytoplasmDevelops extensive RER - increase in protein (antibody) synthesis; transportMore 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)

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An antibody molecule 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 – complementaryto Ag – “Lock and Key”Bind with a specific Ag

Constant region (Fc)Related to class of AbEnables Ab to bind to receptors and attach to cells – e.g. Phagocytes / mast cells

Produced by B lymphocytesLarge globular proteins – Y shaped 4 polypeptide chains (held together by S-S (disulphide) bonds

Hinge region – allows spatialflexibility to branches of the Y shaped variable region of the AbAllows for binding to more than one Ag

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S-S (covalent) bonds hold polypeptide chains (H & L) together

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Mode of Action of Antibodies

Neutralisation and agglutination

-also promotes phagocytosis by the constant region attracting-phagocytes

Immobilise pathogens (bind with flagellum)Destroy bacterial cell wall (lysis)Stop spread

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Communication between cellsCell Signalling in Immune Response

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Some Th cells becomememory cells

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Cell mediated immunity (CMI)

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T cells

Ag presentation by phagocytesReceptors on T cell surface complementary to Ag – specificityClonal selection – Ag selects only those T cells with complementary receptors and activates them Clonal expansion - activated T cells divide by mitosis into a cloneTh 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 parasitesTk – kill abnormal cells with markers for cancerTsup – regulate immune responseMemory T cells – able to differentiate rapidly into Th, Tcyt, Tk cells on stimulation with Ag

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Flow chart to represent the immune response