second line of defence Flashcards
when is second line of defence activated?
when first line of defence is passed
how are extracellular pathogens attacked?
by phagocytes
define: phagocyte
type of white blood cell that engulfs a pathogen by phagocytosis and uses intracellular digestion to destroy it.
3 types of phagocytes?
neutrophils, monocytes and macrophages
describe neutrophils
Found in circulation but migrate into sites of infection in tissues.
First cells to arrive at infection site in response to signals from other cells.
3 lobed nucleus
Short lived
describe monocytes
Found in circulation. When they move into tissues, they differentiate into macrophages.
Long lived
describe macrophages
From differentiated monocytes in tissues.
In addition to phagocytosis, they eliminate dead cells and cell debris, and initiate inflammatory response by secreting cytokines, play a role as antigen presenting cells.
describe how phagocytosis works
• Macrophage ‘s pattern recognition receptors on pm bind to molecular patterns, which recognise it as belonging to a group of pathogens
• Macrophage engulfs the pathogen via phagocytosis, forming a vesicble around it called a phagosome.
• Lysozymes containing digestive enzymes fuse with the phagosome, and release the enzymes into it, which digests the pathogen
o Indigestible material excreted by exocytosis.
o Some partially digested peptides are retained by macrophages and dendrites for the purpose of antigen presentation
how are intracellular pathogens attacked
whole cell containing thme eliminated by natural killer cells by degranulation
describe how NK cells decide whether to kill a cell
- Natural killer cells have killer activating and killer inhibitory receptors.
- All body cells carry ligands that bind to killer activating receptors – which signals to the NK cell to kill the cell.
- However, normal HLA markers bind to killer inhibitor receptors – which makes a signal which blocks the kill signal.
- HLA markers can be abnormal (on cancer cells) or missing (on virus infected cells, which suppress expression of Class I NHA markers); so there won’t be anything that can mind to the killer inhibitory receptors, and the cell will be killed.
describe degranulation
• Granules (vesicles) contain protease enzyme granzyme and the cylindrical protein perforin.
• Granules undergo exocytosis.
• Perforin pierces the plasma membrane to form a pore, then the granzymes enter through them and induce apoptosis
o Important that apoptosis, not lysis, happens, as otherwise the virus would be released into the extracellular fluid and infect other cells.
dendritic cells are the main type of…
antigen presenting cells
define: antigen presenting cell
cells that move antigens to their surface and then display these antigens to other immune cells.
where are dendritic cells typically found?
in tissues like skin, airway linings and gut
how do dendritic cells work?
o Engulfs pathogens via phagocytosis and degrades them; but some peptide fragments are retained, forming antigens
o Antigens are then linked to class II MHC molecules in cytoplasm and then transferred to the cell surface.
o Then, the dendritic cells go to the nearest lymph nodes and present them to naïve helper T cells.
ALSO secrete antiviral cytokenes
where are mast cells usually found?
in tissues near external environment
what do mast cells do?
• IgE antibodies produced by plasma bodies bind to mast cells = primed.
• Mast cells’ antibodies bind to antigen, activating the mast cells and causing degranulation – chemicals like histamine are produced.
o Histamine and other chemicals attract other immune cells to the infection site
o Occurs in inflammation/allergies
define: complementproteins
proteins dissolved in blood plasma
how many types of complement proteins
about 20
describe how inactive comp proteins are changed to active forms
• Exist in bloodstream as dissolved, inactive enzymes, but when they make direct contact with molecules on the surface of a pathogen they are activated (split). The activation of one leads to the activation of others in a cascade effect.
3 effects of complement proteins
o Opsonising pathogens
Opsonisation: complement proteins coat pathogens surface antigens.
Phagocytes have receptors for complement proteins. The cp’s, along with the pathogen, bind do the phagocytes which digest the whole thing.
This makes the pathogen more susceptible to phagocytosis.
o Small complement proteins can diffuse from surface of pathogens and chemically signal to attract immune cells involved in the inflammatory response to the site of infection
o Causes pathogens to lyse
Multiple CP’s form a membrane attack complex, which pierces the cell membrane and enables entry of extracellular fluid, due to osmosis of cell it will swell and lyse.
define: interferons
proteins secreted by some cells after a viral infection
how do interferons work?
they bind to recepotrs on neighbouring cells to produce signals that:
• IgE antibodies produced by plasma bodies bind to mast cells = primed.
• Mast cells’ antibodies bind to antigen, activating the mast cells and causing degranulation – chemicals like histamine are produced.
o Histamine and other chemicals attract other immune cells to the infection site
o Occurs in inflammation/allergies
first thing that happens in the inflammatory response after pathogens enter body?
damaged cells release histamine and cytokines
what does the damaged cells releasing histamine cause?
o Arteriole, capillary bed and venule to dilate
More blood flow causes redness and heat
Increases the permeability of the capillaries; exudate (protein-rich fluid) leaks into infected region – causes swelling, increases pressure on surrounding tissues which activates pain receptors.
• Contains clotting agents which isolate the infection
Immune cells can squeeze through capillary cells to enter infection site.
what does the damaged cells releasing cytokines cause?
attract neutrophils. Following this, other cells are attracted: eg. mast cells and macrophages (which release histamine which amplifies these responses and attract more immune cells)
what are the roles of phagocytes (neutrophils/macrophages?)
phagocytose bacteria/cell debris, resulting in pus formation
deifn:E pus
substance containing dead immune cells (esp. phagocytes, some live cells and cell debris)
what happens when the pathogen is eliminated?
above responses are reversed.
