Cell Recognition + the Immune System Flashcards
What are the defence mechanisms our bodies have against pathogens?
- preventing entry of pathogens by physical + chemical defences (e.g. skin, mucous membranes, tears + saliva)
- inflammation of region invaded by pathogen (non-specific inflammatory response)
- recognising ‘foreign’ cells + targeting any pathogenic cells (specific immune response)
How do are bodies recognise ‘foreign’ cells?
- cells have specific molecules (usually proteins) on their surface, enabling them to be identified
- proteins are often part of phospholipid bilayer (e.g. glycoproteins)
- but glycolipids also act as similar markers
- allows body to recognise its own cells + foreign cells
What do these surface proteins enable the body to recognise?
- pathogens (e.g. bacteria, fungi, viruses)
- abnormal body cells (e.g. cancerous cells)
- toxins (e.g. some pathogens, like cholera, release chemicals into the blood)
- cells from other organisms of the same species (e.g. organ transplants)
What are the surface molecules use by the body to identify cells, vital particles + toxins known as?
- antigens
What is the definition of antigen?
- molecules, present on surface of cells, that generate an immune response by lymphocyte cells when detected in the body
What is an example of the importance of antigens defending against pathogens?
- white blood cells (phagocytes) have surface proteins that act as receptors + bind to antigens on surface of pathogens
- enables pathogens to be engulfed + digested
- antigens found on pathogen can be presented on surface of phagocyte
- recruits other cells of immune system: leads to specific immune response
What does every cell in the human body have to identify it?
- markers
What do antigens allow?
- cell-to-cell recognition
Where are antigens found?
- cell surface membranes
- bacterial cell walls
- surfaces of viruses
What are self antigens?
- antigens produced by organism’s own body cells
- doesn’t stimulate an immune response
What are non-self antigens?
- antigens not produced by organism’s own body cells
- stimulates an immune response
What is antigen variability?
- antigens present on surface of some pathogens change frequently due to genetic mutations
Why does antigen variability pose a problem for the immune system of many mammal hosts?
- bc lymphocytes + memory cells produce a specific immune response as they have surface receptors complementary to only 1 antigen
- meaning when antigen on pathogen changes, lymphocytes + memory cells can no longer bind = no secondary immune response
- host gets infected + suffers from disease again
What are common pathogens that exhibit antigen variability?
- the cold virus
- flu virus
What are the diff. types of pathogen?
- viruses
- bacteria
- fungi
- protozoans
What are phagocytes?
- white blood cells produced continuously in the bone marrow
- stored in bone marrow before being distributed around body in blood
What is the function of phagocytes?
- responsible for removing dead cells + invasive microorganisms
- carry out the non-specific immune response
What are the 2 main types of phagocyte?
- neutrophils
- macrophages
What is phagocytosis?
- process of recognising + engulfing a pathogen
What is the difference between neutrophils (n) + macrophages (m)?
- (m) larger than (n)
- (m) are long-lived cells whilst (n) are short-lived cells
- (n) remain in blood whereas (m) move into organs
How do phagocytes carry out phagocytosis?
- phagocytes move towards pathogen via Chemotaxis (attracted by chemicals released by pathogen)
- receptors on phagocyte attach to antigens on pathogen
- phagocyte engulfs pathogen via endocytosis forming a phagosome
- a lysosome bind to phagosome + releases lysozymes that hydrolyse + digest the pathogen
- phagocyte absorbs products + displays antigens on cell membrane (antigen presenting cell)
What are the 2 types of white blood cell?
- phagocytes
- lymphocytes
What are lymphocytes?
- white blood cells involved in specific immune response
- smaller than phagocytes
- have large nucleus filling most of cell
- produces in bone marrow before birth
What are the 2 types of lymphocytes?
- T-lymphocytes (T cells)
- B-lymphocytes (B cells)
Where do immature T-lymphocytes mature?
- leave bone marrow to mature in thymus
How are T-lymphocytes involved in the cell mediated (cellular immune) response?
- once a pathogen has been engulfed + destroyed by a phagocyte, the antigens are presented on the cell surface (antigen presenting cell)
- mature T-lymphocytes have T cell receptors on their surface specific to 1 antigen that can attach to complementary antigens on the APC
- activating T-lymphocytes to divide by mitosis, to replicate + make a large number of clones that differentiate into 2 main types of T cell
What are the 2 main types of T cell that T-lymphocytes differentiate into?
- helper T cells
- cytotoxic T cells (aka killer T cells)
When will T-lymphocytes bind to an antigen?
- if it’s present on the surface of an antigen-presenting cell
What is an antigen-presenting cell?
- a host cell that’s been invaded by a pathogen + is displaying the antigen on its cell surface membrane
What does an antigen-presenting cell do?
- present antigens from toxins, foreign cells + ingested pathogens
- helps recruit cells of immune system to produce a specific immune response
- once T cell receptor binds to complementary antigen on APC, it becomes sensitised + starts dividing to produce clones
What are e.g.s of cells that can present antigens?
- common = macrophages + dendritic cells
- less common = neutrophils
What is the role of helper T cells?
- assist other white blood cells in immune response
- release cytokines (hormone-like signals) which stimulate:
- maturation of B-lymphocytes into antibody-secreting plasma cells
- production of memory B cells
- activation of cytotoxic T cells: destroys virus infected cells + tumour cells
- an inc. rate of phagocytosis
What is the role of cytotoxic T cells (T killer cells)?
- patrol body in search of antigen-presenting body cells
- attach to foreign antigens on cell surface membranes of infected cells + secrete toxic substances that kill infected body cells, along w pathogen inside
- perforins secreted by cytotoxic T cells pierce cell surface membrane, allowing toxins to enter
Where are B-lymphocytes (B cells) found?
- remain in bone marrow until mature + then spreads through body, concentrating in lymph nodes + spleen
How are B-lymphocytes (B cells) involved in the humoral immune response?
- every B lymphocyte has a specific antibody on their cell surface membrane (B cell receptors) that’s complementary to 1 antigen
- when a complementary B cell receptor binds to an antigen, the B cell takes in antigen by endocytosis + displays it on its cell surface membrane (APC)
- the antigen presenting B cell then binds to a complementary helper T cell receptor, activating B cell to rapidly divide by mitosis [clonal expansion]
- clones then differentiate into plasma cells or memory B cells (primary response)
- most are plasma cells: produce monoclonal antibodies
- memory B cells: long-lived, + divides rapidly into plasma cells if reinfected w same pathogen = produces lots of antibodies rapidly so pathogen destroyed before symptoms occur (secondary response)
What is the definition of an antibody?
- proteins produced by plasma cells, as a part of the immune response, which binds to antigens
What is the structure of an antibody?
- quaternary structure protein (4 polypeptide chains)
- 2 heavy chains (long) bonded to 2 light chains (short) by disulphide bonds
- has a constant region
- has a variable region (antigen binding site = specific + complementary)
What is agglutination?
- a complementary antibody binds to an antigen on a pathogen (antigen-antibody complex)
- antibodies: slightly flexible = can bind to 2 antigens + clump them together
- either neutralises pathogen or makes it easier for phagocytes to locate + destroy
What do B cells differentiate into?
- plasma cells
- memory B cells
