Cell recognition and the immune system

which cells can stimulate an immune

response

• pathogens

• cells from other organisms of the same species

• abnormal body cells

• toxins

defence mechanisms against pathogen

Physical barriers (skin, mucous membranes) + Chemical barriers (stomach acid, saliva, tears containing enzyme lysozyme which destroys bacteria)

→ Prevent entry of pathogens

non specific immune response (response is immediate and same for all pathogens)

• Inflammation (swelling and heating) of region invaded by pathogen (non-specific inflammatory response)

• phagocytosis

specific immune response (response is slower and antigen specific so response specific to type of pathogen)

• Recognising ‘foreign’ cells (humoral response, cell mediated response)

Antigen

foreign protein that stimulates an immune response and production of antibody

where r antigens found

• cell surface membranes

• bacterial cell walls

• surfaces of viruses

  Some glycolipids and glycoproteins on the outer surface of cell surface membranes act as antigens

2 types of antigens

• self antigens → produced by the organism's own body cells → immune system does not recognise as foreign antigens → do not stimulate an immune response

• non-self antigens → not produced by the organism’s own body cells→immune system recognises as being foreign → stimulate an immune response

Antibody

A protein (immunoglobulin) specific to an antigen produced by B cells

(Secreted by plasma cells)

antigen variability

pathogen dna can mutate frequently

if genetic mutation occurs in gene which codes for antigen, shape of antigen will change

→ lymphocyte cannot bind

→ previous immunity to pathogen no longer effective bc memory cells in blood complimentary to old antigen shape so cannot bind to mutated antigen shape

→ means no secondary immune response → will suffer from disease again

→ eg influenza virus

phagocytosis

phagocyte moves towards pathogen

phagocyte attaches to the pathogen by binding to its antigens

phagocytes engulf pathogen

vesicle (phagosome) form around pathogen

phagosme and lysosomes fuse

lysozyme digest pathogen by breaking it through hydrolysis

antigens presented on surface of phagocyte (becomes antigen presenting cell)

types of white blood cells

• lymphocytes (specific) → B lymphocytes (B cell), T lymphocytes (T cell)

• phagocytes (non specific)

B lymphocytes (B cells)

produced and matured in bone marrow

→ humoral immunity (immunity involving antibodies that r present in blood plasma

T lymphocytes (T cells)

produced in bone marrow

mature in thymus glands

→ cell mediated immunity (immunity involving body cells )

antigen presenting cell

host's cells that has been invaded by a pathogen and is displaying the antigen on its cell surface membrane

receptor of the T cell binds to specific complementary antigen on APC, triggering immune response

Cell mediated response

response to foreign antigens presented on the surface of antigen-presenting cells

1. Pathogens engulfed by cell (eg phagocyte) and its antigens presented on surface of cell (antigen presenting cell)

2. Receptors on a specific helper T cell bind exactly onto the particular antigens

3. This attachment activates helper T cell to divide by mitosis + form clones of genetically identical cells

4. cloned T cells:

   
   develop into memory t helper cells → enable a rapid response to future infections by the same pathogen (secondary immune response)

   
   stimulate phagocytes → to engulf pathogens by phagocytosis

   stimulate B cells → to divide by mitosis and secrete their antibody

   activate cytotoxic cells (killer T cells) → attaches to abnormal or infected and releases protein called perforin which will form holes in the cell membrane, destroying cell

humoural response

how do lymphocytes recognise cells

each lymphocyte can recognise a diff shaped antigen on self cells

lymphocytes made when foetus

→ when foetus in womb, unlikely to be exposed to any cells other than self cells

→ lymphocytes complementary to antigens on self-cells

what is active immunity

when our own immune system is responsible for protecting us from a pathogen

what is passive immunity

when we are protected from a pathogen by immunity gained from someone else eg breastfeeding

what r pathogens

micro-organisms that cause diseases → virus, bacteria, fungi, protists

when r lymphocytes made

during foetal development → some lymphocytes will undergo a process of programmed cell death (apoptosis)

these r usually the type that have receptors that fit the body’s own cells (self)

leaving only remaining lymphocytes which have receptors to recognise foreign material ( non self)

in adults, lymphocytes made in bone marrow

plymphcytes recognise pathogens, not respond to pathogens

lymphocytes already exist→ not originally produced BECAUSE there is infection

after a pathogen gains entry to the body it is often a number of days before bods immune system begins to control it, suggest a possible reason why

bc white blood cells need to divide and replicate by mitosis to create more to control pathogen

y is it not accurate to say that body takes days to “respond ‘to the pathogen

because it takes time for phagocytes to recognise the pathogen before it can formulate a response as it need to replicate and divide by mitosis

phagocytosis is the process which pathogens r taken up and destroyed by white blood cells

these white blood cells r continually produced from stem cells in bone barrow

they r stored in bone marrow and released into the blood to engulf and digest foreign bodies (pathogens)

there r 2 types of phagocytes : neutrophils and macrophages (monocytes)

receptors → non specific

antibodies → specific

what is antigen

• An antigen is any part of an organism or substance that is recognised as non-self (foreign) by the immune system and stimulates an immune response

• Antigens are usually proteins that are part of the cell-surface membranes or cell walls of invading cells e.g. microorganisms or abnormal body cells like cancer cells

• The presence of an antigen triggers the production of an antibody as part of the body's defence system

lymphocytes

immune responses eg phagocytosis → not specific and occur whatever the infection

the body also has specific responses that react to specific antigens

these r slower in action at first but provide long term immunity

this specific immune response depends on a type of white blood cell called lymphocyte

lymphocytes produced by stem cells in bone marrow

typ

types of lymphocytes

b lymphocytes (B cells) → mature in bone marrow

→ associated w humoral immunity (immunity involving antibodies that r present in blood plasma

t lymphocytes (T cells) → mature in thymus glands

associated w cell mediated immunity (immunity involving body cells )

t- cells cell mediated bc respond to antigens presented on cells (APC→ antigen presenting cells), and don’t respond to antigens detached from cell and in body fluid eg blood

how can antigen presenting cells (APC) appear

APC→ any cell that presents a non self antigen on surface

eg: on infected body cells have viral antigens on their surface

a macrophage (phagocyte) with has engulfed and destroyed a pathogen will present the antigens on their surface

cells of translated organs will have different shaped antigens on their surface compared to your self- cell antigens

cancer cells will have abnormal shaped self- cell antigens

cell mediated immunity

helper T cells have receptors on their surface which can attach to the antigens on APC

once attached, tis activates the helper T cells to divide by mitosis to replicate and make large numbers of clones

cloned helper T cells differentiate into different cells

some remain as helper T cells and activate b lymphcytes

some stimulate macrophages to perform more phagocytosis

some become memory cells for that shaped antigen

some become cytotoxic T cells (killer T cells)

b lymphocytes the humoral response

lymphocytes r white blood cell involved in specific immune response

plasma cells make antibodies

plasma cells

secrete antibodies into blood plasma

cells survive only a few days but each can make around 2000 antibodies every second

these lead to destruction of antigens

the plasma cells r therefore responsible for the immediate defence of the body against infection

production of antibodies and memory cells → primary immune response

memory B cells do not make antibodies → divide by mitosis and make plasma cells rapidly if they collide w antigen they have previously encountered

reuses in large numbers of antibodies beg produced rapidly do pathogen desytroed before symptoms occur → active immunity

describe how b lymphocytes r involved in the immune respponse

antigens in blood collide w complimentary antibodies on b lymphcuyes

b lm]ymphcoyte takes in antigen by endocytosis then presents antigen on cell surface membrane

B cells that go through mitosis to produce clones (clonal expansion) and differentiate to either plasma cells to produce antibodies or memory B cells that remain in body and rapidly produce plasma when come in contact w antibody again

mark scheme

antigens presented by macrophages

b lymphocytes w complimentary antibody on its surface binds to antigen

b lymphocytes divide by mitosis and form clones

plasma cells release antibodies into blood

and memory cells formed