cell recognition and the immune system
cell recognition
-barriers to infection:
-non-specific defences: response is immediate and the same for all pathogens. eg, physical barriers (skin), phagocytosis
-specific defences: response is slower and specific to each pathogen. eg, cell mediated response (T-lymph), humoral response (B-lymph)
-lysozyme in tears and other secretions
-removal of particles by cilia
-skin surface, fatty acids and normal flora
-stomach - PH2
-flushing of urinary tract
-antigens:
-"any part of an organism or substance that is recognised as non-self or foreign by the immune system and therefore stimulates a immune response by lymphocytes"
-self and non-self refers to foreign or non-self particles being attacked by native or self particles
-antigens act as markers on cell surface membranes, unique to each person
-found in abnormal body cells such as:
-cancer cells
-virus host cells
-antigen presenting cells
-pathogens
-phagocytosis:
-white blood cells that can carry out phagocytosis are phagocytes
-they are a non-specific immune response
-they engulf and digest foreign materials such as pathogens
-antigen presentation:
-at the end of phagocytosis the phagocyte will display the antigens of the pathogen on its cell surface membrane - antigen presenting cells
-important for the lymphocytes to carry out their roles
T-lymphocytes and cell mediated response
-specific immune response:
-response is specific to particular antigens
-slower, can provide long-term immunity
-involves lymphocytes:
-B-lymphocytes provide humoral immunity
-T-lymphocytes provide cell mediated response
-antigen presenting cells:
-T-lymphocytes can only respond to an antigen that is present on body cells such as:
-infected cells
-phagocytes
-cancerous cells with altered antigens
-transplanted cells
-T-helper cells:
-there are a vast number of different Th-cells each responding to different antigens
-receptors on Th-cells will fit exactly to the specific antigen
-the Th-cell then will divide rapidly by mitosis to form clones
-cytotoxic T-cells or Tc-cells:
-Tc-cells are responsible for destroying harmful abnormal cells and infected cells by secreting cytotoxic chemicals such as perforin, a protein that makes holes in cell membranes
cell recognition 2
-antibodies:
-produced by B-lymphocytes
-proteins made of 4 polypeptide chains (quaternary structure)
-antibodies are specific to the type of antigen
-the specific antigen binding site of an antibody fits precisely to the antigen to form a antibody-antigen complex
-every antibody has a different shaped binding site
-antibodies do not destroy antigen directly, but prepare them for destruction. eg:
-agglutination of bacterial cells - causes them to clump together to increase efficiency of phagocytosis, they also act as markers and stimulate phagocytes
-B-lymphocytes and humoral immunity:
-B-lymphs provide humoral immunity by producing antobodies
-approx. 10 mill types of B-cell and each one is specific to the antigen
-when a specific B-cell encounters its antigen it is taken into the cell by endocytosis
-it is then presented on the surface
-Th-cells attach to the antigens and activate the B-cell that then produce the antibody specific to the antigen - monoclonal antibody
-2 types of B-cell:
-plasma cells that secrete antibodies - primary response
-memory cells responsible of secondary response
-why don't lymphocytes destroy our own cells?:
-lyphocytes begin to develop in the foetus where they come into contact with self cells almost exclusively
-any cells that have receptors to these cell antigens are destroyed or suppressed
-in adults, lymphocytes produced in bone marrow also initially only encounter self antigens
-any that show no immune response undergo programmed cell death before they mature
the primary and secondary immune response
-primary responses slow so the pathogen can establish an infection
-secondary response is fast so pathogen is killed before infection can take place
-antigenic variation:
-although a secondary immune response has been developed we can still suffer from the disease
-this is from antigens changing on the cell surface
-this is achieved by antigen variability: mechanism of immune evasion wherein a pathogen changes its surface antigens to avoid detection
-antigen variability in bacteria:
- some bacteria can change the pili by a changing their subunits
-antigen variability in viruses:
-due to mutagenic acclumation
-mutation leads to shape change, viruses have a high mutation rate
-antigenic shift:
-surface antigens reassort between different viruses making new antigens
-pandemics are result of antigenic shift
-zoonotic infection is due to an animal and human strain mutating