AS Biology F212: Immunity

Primary defences

• These prevent the pathogen from entering the body
• These mechanisms have evolved through selection and are considered as adaptations

Barriers to entry

• The skin
• the skin covers the body surface providing a physical barrier that most pathogens find hard to penetrate
• Keratinocytes
• cells produced by mitosis at the base of the epidermis
• they migrate to the skins surface
• they dry out and the cytoplasm is replaced by keratin = keratinisastion
• this prcess take about 30 days
• by the time they reach the surface they are dead and everntally slough off
• Mucous membrane
• produced by the epithelial layer traps pathogens
• epithelial layer contains goblet cells
• these produce mucus that lines the airways trapping pathogens
• ciliated cells move the mucus up the oesophagus so it can be swallowed
• pathogens are killed by the acidity of the stomach (pH 1-2) which denatures their enzyme
• Mucus membranes are also found in the gut, ears and nose
• Hydrochloric acid in the stomach provides a low pH that the enzymes of most pathogens are denatured and therefore the organisms are killed

Secondary Defence

• Phagocytes (non-specific)
• Neutrophils - most common
• have multi-lobed nucleus and are produce in the bone marrow
• they are found in the blood and tissue fluid and may also be found on epithelial surfaces such as the lungs
• short lied but are released in large numbers
• Macrophage - larger than neutrophils
• manufactures in the bone marrow
• travels in the blood as monocytes
• they tend to settle in the body organs particularly the lymph nodes
• here they develop into macrophages
• they plan an important part in specific response to pathogens
• The role of macrophages
• infected cells release histamine
• this attract neutrophils
• it also causes capillaries to become more leaky
• as a result more fluid leaves the capillaries in the area of infection
• so more fluid passes the lymphatic system
• this leads the pathogen towards the macrophages in the lymph nodes
• How phagocytes work
• they engulf and destroy pathogenic cells
• these have chemical markers on their outer membranes = antigens
• these are recognised as non-self or foreign
• these are specific to pathogen
• Phagocytosis
• once bound the phagocytes envelopes the pathogen by folding its membrane inwards
• the pathogen is trapped within a phagosome
• lysosome fuse with phagosome and release enzymes lysins
• these digest the pathogen producing harmless products that can be absorbed

Specific immune response

T and B lymphocytes

• these have receptors that are complementary to the foreign antigen
• this antigen may be attached to the pathogen or on the surface of the host cell
• when antigen is detected, the lymphocyte is activated

"Describe the changes that occur to T lymphocytes during an immune response. Explain the roles of T lymphocytes in fighting infection by a pathogen, such as a virus"

• reference to antigen presentation - described
• receptors on T cell surface or complementary to the antigen
• reference to specificity in context of T cells
• clonal selection - described
• clonal expansion/T cells divide by mitosis
• T helper cells release cytokines
• stimulates B cells to divide/clone/differentiate
• stimulate macrophage to carry out phagocytosis
• T/Killer cells search and kill infected host cells
• secrete enzymes
• named enzyme
• active immunity
• memory T cells/immunological memory
• secondary response - more rapid

Antigen

• Usually large proteins or glycoproteins
• specific shape
• the antigen is specific to the pathogen
• a foreign antigen stimulates the production of antibodies = an immune response
• there is one type of antibody for one type of antigen

Antibodies

• produced by lymphocytes (WBC)
• are large proteins also known as immunlglobulins
• they have specific shape tat is complementary to particular antigen
• these antibodies are specific to the antigen
• therefore the antibody is specific to the pathogen
• antibodies attach to antigens and render them harmless

Antibody structure

• the constant region is the same on all antibodies
• this allows the attachment of phagocytic cells during phagocytosis 
• the variable region has a specific shape and differs from one type of antibody to another
• this is due to its amino acid sequence
• this makes it complementary to a particular antigen
• this allows it to bind to the antigen
• the hinge region allow flexibility 

Antigens, antibodies and phagocytosis

• the presence of a foreign antigen can trigger the production of antibodies
• antibodies (proteins) in our blood attach to foreign antigens (antigen-antibody complex)
• phagocytes have membrane bound receptor proteins
• these can bind to the antibody antigen complex
• allows the recognition of the pathogen

How antibodies work - neutralisation

• the antigen in the pathogens cell surface membrane may be used as a binding site
• this would allow it to bind to host cells
• if the antibody blocks this binding site then the pathogen cannot bind to the host cell
• this is called neutralisation 

Agglutination

• some antibodies are larger than those previously described
• they resemble several Y shaped molecules attached together with many specific variable region
• each one can bind to an antigen on a pathogen
• the attachment to many pathogens at the same time is called agglutination
• the pathogens cannot enter host cells

Producing antibodies

• Infection = antibody production
• It takes a few days before antibody levels are high enough to successfully combat the infection
• This is the primary immune response
• antibodies do not stay in the blood
• if the body is infected a second time by the same pathogen, the antibodies must be made again
• however, the production is faster and the concentration is higher
• this is the secondary immune response

Communication between cells

Cell signalling

• Communication is achieve through sell surface molecules and through the release of hormone like chemicals called cytokines
• the target cell must have a cell surface receptor

What information is communicated

• Identification - the pathogen carries antigen that identify it as foreign 
• Distress signals - these are produce when a cell becomes infected by a pathogen
• lysosomes breaks down the infecting pathogen
• parts of the pathogen end up attached to the host cells plasma membrane (antigen presentation)
• these can act as a distress signal that can be detected by other cells
• or they can act as markers so the infect host cell can be destroyed by T killer cells
• Antigen presentation
• macrophages engulf pathogens by phagocytosis
• they do not fully digest them
• they incorporate the antigen into their cell surface membrane 
• they become known as antigen-presenting cells
• its function is to find the lymphocyes that can neutralise that particular antigen
• Instructions
• chemicals called cytokines act as instructions to target cells
• they bind to specific receptors
• this causes the release of second messengers inside the cells
• e.g. macrophages release monokines to attract neutrophils
• macrophages release monokines to stimulate B lymphocytes to release antibodies
• T and B lymphocytes release interleukins which stimulate the proliferation and differentiation of T and B lymphocyte
• Many cells release interferon which can inhibit virus replication and stimulate killer T cells