Specific defences against disease - Antibodies, Antigens

immune system

Composed of cells and proteins that protect against foreign organisms, a range of alien chemicals, cancerous and other abnormal cells

Immune response

A homeostatic mechanism when microorganisms or foreign substances enter the body, helps deal with invasion and restore internal environment to its normal condition

Key cells

lymphocytes - b-cells, t-cells

B-cells

produced in bone marrow and mature there to become B-cells and move to lymphoid tissue. Each has a receptor for a particular antigen and can respond to it

T-cells

Also produced in bone marrow but mature in thymus to become T-cells before moving to lymphoid tissues

Parts of immune response

Humoral (antibody-mediated): production of antibodies by B-cells which circulate in the body to attack pathogens

Cell-mediated: due to T-cells, involves forming special lymphocytes that destroy invading pathogens

Antigens

any substance capable of causing a specific immune response, e.g virus particles, whole microorganisms, part of a bacterium, toxins and others

Self-antigens

Large molecules produced in a person's own body that do not cause an immune response.

Non-self antigens

Foreign compounds that can trigger an immune response

Antibodies

Specialised proteins that are produced in response to a non-self antigen, belongs to group of proteins called immunoglobulins. Circulate in blood, lymph and extracellular fluid and attach to antigens on pathogen

Antibody lock-and-key model

Antibody produced can combine with an antigen to form antigen-antibody complex specific to antibody

Antigen presenting cells

Recognise and respond to non-self antigens that enter the body, include dendritic cells, macrophages and undifferentiated B-cells

Antigen-presenting cells functions

- Detect the presence of a non-self antigen

- engulf the pathogen and digest it, producing small fragments that move to the surface of the cell

- presenting antigen to lymphocytes

- when a pathogen is consumed by macrophage/dendritic cell they become an APC

Antibody mediated immune response

Humoral response involves production and release of antibodies into blood and lymph, provides resistance to viruses, bacteria and bacterial toxins before these microorganisms/substances enter body's cells.

Lymphoid tissue

Contains many lymphocytes and macrophages, found mostly in lymph nodes

Activation of B-cells

Activated by APCs and helper T-cells. Helper T-cells produce cytokines in response to antigens, causes cloning of helper T-cells. Clones then produce a cytokine that activates B-cells

When B-cells are activated

Enlarge and divide via mitosis to produce clones, most of which become plasma cells which secrete specific antibodies that attach to active site of the antigen

Remainder of B-cells

Become memory cells which allow response to occur more rapidly, should antigen invade body again (secondary response)

Primary response

First exposure to an antigen, fairly slow and often takes days to build up large amounts of antibodies.

As plasma cells begin to secrete antibodies, level increases in blood plasma

Once it reaches a peak, it begins to decline, leaving immune system with memory of particular antigen

Secondary response

Subsequent exposure to same antigen, response is faster due to activity of memory cells. Plasma cells are formed very quickly and antibody levels in blood plasma rise rapidly - antibodies are so quick, antigen has little opportunity to exert any noticeable effect on body

How antibodies work

Form antigen-antibody complex, they can:

- combine with foreign enzymes/bacterial toxins or inactivate them by inhibiting reaction with other cells or compounds

- bind to surface of viruses and prevent them from entering cells

- coat bacteria so it's more easily consumed by phagocytes

- cause particles such as bacteria, viruses, or foreign blood cells to agglutinate

- dissolve organisms

- react with soluble substances to make them insoluble

Cell-mediated response

Provides resistance to the intracellular phase of bacterial and viral infections, important in fighting whole cells, e.g providing resistance to fungi and parasites and in rejecting foreign-tissue transplants. Occurs when T-cells are stimulated, resulting in production of killer T-cells and helper T-cells as well as memory cells

T-cells features

Occur in different areas of lymphoid tissue to B-cells, only respond to one antigen

Activation of t-cells

APC stimulates helper t-cells through release of cytokines, causing them to release their own cytokines. Causes helper T-cells programmed for antigen to activate. Activated T-cells enlarge and divide via mitosis (making clones)

T-cell clones differentiation

More helper T-cells, memory T-cells that remain in lymphoid tissue, killer T-cells and suppressor T-cells

Helper T-cells

enhances activity of T-cells and B-cells by binding to APC by

- causing cytokines to be secreted

- causing lymphocytes at site of infection to become sensitised and activated (intensifies response)

- attracting macrophages to the place of infection so that they can destroy antigens via phagocytosis

Killer T-cells

Migrate to site of infection, deal with antigen, attach to invading cells/host cells and secrete substance e.g cytotoxins that destroy pathogen

Memory T-cells

Remain in lymphoid tissue to remember antigen/pathogen if encountered again (secondary response)

Suppressor T-cells

Slow down immune response once pathogen is dealt with

Immunity

Resistance to infection by invading microorganisms

Natural immunity

Occurs without human intervention

Artificial immunity

Results from giving people an antibody or antigen

Passive immunity

When someone is given antibodies produced by someone else so immunity is established immediately. Short-lived -only lasts days until antibodies are broken down and excreted

Natural passive immunity

When antibodies from mother pass across placenta to a developing fetus or through breast milk

Artificial passive immunity

person is injected with antibodies to combat a particular infection

Active immunity

Results when body is exposed to a foreign antigen and manufactres antibodies itself. Prolonged as memory of antigen persists

Natural active immunity

Can result from an actual attack from disease

Artificial active immunity

An injection of antigens associated with the disease, i.e vaccine