3rd Line of Defence

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The Immune Response

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� The main components of the bodys main

immune response are:

Lymphocytes: B cells

T cells

Antibodies

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� There are manydifferent types of 

WBC

s� Lymphoctyes are non-

phagocytic whiteblood cells

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� All the differenttypes of bloodcells, including

lymphocytes, arisefrom stem cellsmade in the bonemarrow

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� The 2 different types of lymphocytes

responsible for the immune response in

humans are: B cells (humoral or antibody mediated response)

T cells (cell mediated response)

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� Each B cell has an antibody on its surface that

can bind with a particular chemical shape

� If a B cell collides with a molecule, an antigen,that binds its antibody it becomes activated

and goes to the lymph node to rapidlyreplicate

� This is called clonal expansion

� Most cloned cells specialise to become

plasma cells

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� A plasma cell will make and release aparticular antibody which exactly binds with

the antigen that triggered the response

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� Antibodies areproteins found in blood

plasma that can bindwith and help toneutralise an antigen

� Antibodies are highly

specific for the antigenthat stimulated theirsynthesis and release

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� An antibody can react with an antigen in anumber of ways: Combine with the antigen to cover the active site so it

has no effect on the body Dissolve parts of the cell walls of bacteria, destroying

them

Neutralise the toxins made by the antigen

Cause bacteria to clump together, making thebacteria less active and more easily taken up by the

lymph system

Make the antigen more susceptible to phagocytosis

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� Can live for many years in the body

� They carry the same antibody so that ever an

antigen (on a pathogen or foreign chemical)enters the body again, it will immediately be

recognised, and cloning and antibodyproduction can begin immediately

� The body can begin destroying the pathogenbefore it has time to cause an infection

� ie. You are immune to that pathogen!

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� Made in the bone marrow� Mature in the thymus gland, situated under the

sternum

� Constantly circulating through the blood to thelymph nodes and spleen and then back to theblood

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� One component of the 2nd line of defenceinvolves macrophages acting on pathogens

by engulfing and destroying them byphagocytosis� The macrophage will then display pieces of 

antigen from the destroyed pathogen on its

surface on complex molecules called majorhistocompatibility complexes (MHCs)� B cells can also present antigens onMHCs on

their surface to T cells

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� T cells recognise theseantigens by interactingwith these macrophagesandB cells

� These lymphocytes canthen launch a

coordinated attack todestroy any remainingantigen

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� T cells can differentiate into different kinds of 

cells, each with different roles:

Killer, or cytotoxic, T cells Memory T cells

Helper T cells

Suppressor T cells

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� Have a T cell receptor (TCR) on the surface of each cell

�

TC

Rs work with body fluid proteins calledmajor histocompatibility complexes (MHCs)� Each T cell searches the surfaces of cells

throughout the body for anMHC to match

with its TRC� If it finds a match, T cell cloning begins to

make killer cells that destroy those cells� Activated by interferons

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� Use TCRs to recogniseMHCs on B cells,macrophages, and other white blood cells,

and produce chemicals called cytokines inresponse

� Cytokines affect the recognised cell, helping

other lymphocytes to destroy it, as well as

stimulating the production of more B cellsand T cells and coordinating their response

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� Turn off the productionof other lymphocytes(bothB and T cells)when the antigenshave been removed

� Also play a role in

protecting the bodyfrom immune attackson itself 

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� Retain the ability to recognise the original

invading antigen so that a subsequent

invasion can be dealt with quickly� Take no further part in the attack but remain

in the body ready to respond more quickly toany future antigenic molecules

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