IMMUNITYIMMUNITY

The Immune SystemThe Immune System

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• Physical, chemical Physical, chemical and cellular and cellular defences:defences:– Epithelia that cover Epithelia that cover

the airwaysthe airways

– Hydrochloric acid in Hydrochloric acid in the stomachthe stomach

– Blood clottingBlood clotting

– White blood cellsWhite blood cells

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• ImmunityImmunity – the protection against – the protection against disease provided by the body’s defence disease provided by the body’s defence or immune systemor immune system

• AntigenAntigen – any large molecule (e.g. – any large molecule (e.g. protein) recognised by the body as protein) recognised by the body as foreignforeign

• AntibodyAntibody – a protein made by the – a protein made by the immune system in response to the immune system in response to the presence of an antigen and targeted presence of an antigen and targeted specifically at itspecifically at it

• Immune responseImmune response – lymphocytes – lymphocytes respond to the presence of a pathogen respond to the presence of a pathogen in the body by producing antibodiesin the body by producing antibodies

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• Cells of the immune system are Cells of the immune system are produced in the bone marrowproduced in the bone marrow

• 2 groups of these cells involved 2 groups of these cells involved in defence:in defence:– Phagocytes (neutrophils and Phagocytes (neutrophils and

macrophages)macrophages)– LymphocytesLymphocytes

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PhagocytesPhagocytes

• Scavengers produced in bone Scavengers produced in bone marrow, removing any dead cells as marrow, removing any dead cells as well as invasive microorganismswell as invasive microorganisms

• NeutrophilsNeutrophils::– 60% of the white blood cells60% of the white blood cells– Tissue ‘patrol’Tissue ‘patrol’– Released in large numbers from Released in large numbers from

stores but are short-livedstores but are short-lived

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• MacrophagesMacrophages::– Larger than neutrophilsLarger than neutrophils

– Found in organs (lungs, liver, spleen, Found in organs (lungs, liver, spleen, kidney and lymph nodes)kidney and lymph nodes)

– Leave bone marrow and travel in blood Leave bone marrow and travel in blood as monocytesas monocytes

– Long-lived Long-lived

– Initiates immune responses (cut Initiates immune responses (cut pathogens up to display antigens) pathogens up to display antigens)

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Phagocytosis Phagocytosis

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LymphocytesLymphocytes• Smaller than phagocytes with large Smaller than phagocytes with large

nucleusnucleus• 2 types that are produced before birth in 2 types that are produced before birth in

the bone marrow:the bone marrow:– B lymphocytesB lymphocytes (B cells) (B cells)

• Remain in bone marrow until matureRemain in bone marrow until mature• Spread throughout body (lymph nodes and spleen)Spread throughout body (lymph nodes and spleen)

– T lymphocytesT lymphocytes (T cells) (T cells)• Leaves bone marrow and mature in Leaves bone marrow and mature in thymusthymus• Coordinates the immune response, stimulating B cells Coordinates the immune response, stimulating B cells

to divide and then secrete antibodies into the bloodto divide and then secrete antibodies into the blood• Seek out and kill any of the body’s own cells that are Seek out and kill any of the body’s own cells that are

infected with pathogeninfected with pathogen

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B lymphocytesB lymphocytes

• Each B cell makes just one type of Each B cell makes just one type of antibody moleculeantibody molecule

• Cell divides to give small number of Cell divides to give small number of cells that make the same antibody cells that make the same antibody ((cloneclone))

• Antibody remains in plasma membrane Antibody remains in plasma membrane and part of each antibody forms a and part of each antibody forms a protein receptorprotein receptor

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• Memory cellsMemory cells::– Basis of Basis of immunological memoryimmunological memory

(remain circulating for a long time, (remain circulating for a long time, often lasting a lifetime)often lasting a lifetime)

– Rapidly divide and develop into Rapidly divide and develop into plasma cells and more memory cells plasma cells and more memory cells upon reintroduction of antigenupon reintroduction of antigen

– Infection can be destroyed and Infection can be destroyed and removed before any symptoms removed before any symptoms developdevelop

– Leads to a faster secondary responseLeads to a faster secondary response

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Antibodies Antibodies • Globular glycoproteinsGlobular glycoproteins• Plasma protein (Plasma protein (immunoglobulinsimmunoglobulins))• Consists of:Consists of:

– 4 polypeptide chain (2 heavy and 2 light)4 polypeptide chain (2 heavy and 2 light)– Disulphide bridgesDisulphide bridges– 2 identical antigen binding sites2 identical antigen binding sites– Variable region (specific shape that binds Variable region (specific shape that binds

to just one antigen)to just one antigen)– ‘‘hinge’ region (flexibility for antibody to hinge’ region (flexibility for antibody to

bind around antigen)bind around antigen)

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T lymphocytesT lymphocytes

• Have specific cell surface receptors (T cell Have specific cell surface receptors (T cell receptors)receptors)

• Activated when encounter antigen in Activated when encounter antigen in contact with another host cell (e.g. contact with another host cell (e.g. macrophage or invaded body cell)macrophage or invaded body cell)

• 2 types:2 types:– T helper cellsT helper cells

• Secrete cytokines – stimulate B cells to divide and Secrete cytokines – stimulate B cells to divide and stimulate macrophage to carry out phagocytosis stimulate macrophage to carry out phagocytosis more vigorouslymore vigorously

– killer T cells (T cytotoxic cells)killer T cells (T cytotoxic cells)• Search invaded body cells, attach themselves and Search invaded body cells, attach themselves and

secrete toxic substances (hydrogen peroxide)secrete toxic substances (hydrogen peroxide)

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