B cells in the immune responseBefore birth in the bone marrow immature B cells divide by mitosisStill in the bone marrow each B cell maturesBecause they mature in the bone marrow they are called B cellsMaturation involves the production of receptor molecules which are expressed on the plasma membrane of the B cellEach B cell has a different receptor which will bind with a different antigenMature B cells circulate and concentrate in the lymphoid tissuesBy birth there are millions of different B cells each with a specific receptor.

Production of antibodyWhen B cells are activated they differentiate into plasma cellsPlasma cells produce large quantities of antibody moleculesThe antibody produced by the plasma cell has the same antigen specificity as the receptor on the surface of the B cellPlasma cells have an extensive network of rough endoplasmic reticulum for antibody (protein) productionPlasma cells are only short lived and after several weeks their numbers decrease.

Clonal selection , expansion and differentiationA pathogen with antigens on its surface enters the bodyOnly one of these B cells has a receptor that matches the antigen. This B cell is selected.The selected B cell divides by mitosis to form a clone. This is clonal expansion.Some of the clone of B cells differentiate into plasma cells and secrete antibodyOthers turn into memory cells which will persist in case the antigen enters the body a second time.

Now watch it happen!

T cellsT cells are made in the bone marrow before birthThey then migrate to the thymus gland (thats why theyre called T cells) and then mature in exactly the same way as B cellsA mature T cell will express receptors on its surface which can bind antigenSpecific T cells are activated (clonal selection) when they come across macrophages which have processed the appropriate antigens. The macrophages secrete cytokines to activate the T cells to undergo clonal expansionThe T cells may be T helper cells which express a receptor called CD4 or cytotoxic T cells which express CD8After activation some T cells will differentiate into T memory cells which are long lived. T suppressor cells are also produced which close down the response after the antigen is eliminated.

Role of the T helper cell in the activation of B cells In order for the B cell to undergo clonal selection and expansion as explained before, help from an activated T cell is required.Macrophages take up and process antigensThey present these on their plasma membranes in conjunction with MHC proteinsT cells with complimentary antigen receptors on their surfaces are stimulated to undergo clonal expansion after binding with the processed antigen due to cytokines released by the macrophageThe result will be a clone of activated T cells specific for that antigen

The T helper cells which have been activated then come into contact with B cells which have also processed antigenThe T cells bind to the surface of the B cells and secrete cytokines which stimulate the B cells to undergo clonal expansion and differentiation into plasma cells

Summary

Role of T cells in cell mediated immunityAntigens are ingested and presented on the surface of macrophages.If a T cell with a CD8 receptor comes across a macrophage which has processed the appropriate viral antigen it will bind to the macrophage.The macrophage will activate the T cell by secreting cytokines. The T cell will then undergo clonal expansion.The result will be a clone of activated cytotoxic T cells or T killer cells

Killing by cytotoxic T cellsOnce the killer T cell is activated it may come across body cells infected by virusThe body cell will have processed the viral antigens and they will be expressed on its surfaceThe killer cell will bind to the infected body cell via its CD8 receptor and will be activated to kill the infected cellThis is accomplished by the killer cell using perforin proteins to puncture the cell membrane and then injecting toxic enzymes which will lead to cell death or apoptosis.

Immunity and cancerWhen normal cells turn into cancer cells some of their surface antigens changeCancer cells often stop expressing the normal antigens of the major histocompatability complex (MHC)These changes can activate cytotoxic T cells to attack and kill the cancer cellsAccording to one theory cells of the immune system are continually patrolling and eliminating cells that undergo malignant transformation.For animations of the immune response go to www.whfreeman.com/thelifewire6e/content/ch19ch/19xe04.htm

AllergiesAsthma, hayfever and eczema are allergic reactions.These occur when the immune system reacts to harmless substances called allergens On first exposure to the allergen IgE antibodies are producedThese bind to receptors on mast cellsThe body is now sensitised to the allergenOn second contact with the allergen the IgE molecules are cross-linkedThis causes the mast cell to release histamine and inflammation occurs

AsthmaThis can be a severe disease and can be fatalPeople with asthma have airways that are nearly always inflamedDuring an asthmatic attack this inflammation worsens due to goblet cells secreting large amounts of mucusThe smaller airways become blocked with fluid The muscle surrounding the trachea, bronchi and bronchioles contracts narrowing the airwayBreathing becomes difficult and wheezing occursAsthma can be treated using drugs which relax the smooth muscle these are called bronchodilatorsHouse dust mites, their faeces, animal skin and pollen can all be allergens.See www.schoolscience.co.uk for good info and animations on asthma