4.2 Immunity Flashcards
4.2 Immunity
Defence Mechanisms
The body has a range of defences to protect itself from pathogens. There are two main types:
•Non-specific mechanisms do not distinguish between one type of pathogen and another, but respond to all of them in the same way. These mechanisms act immediately and take two forms:
1. A barrier to prevent entry of pathogens
- Phagocytosis
•Specific mechanisms do distinguish between one pathogen and another. They respond less rapidly but provide long lasting immunity. The response involves a type of white blood cells called and lymphocyte which can take two forms:
1. Cell mediated immunity involving T lymphocytes
- Humoral response involving B lymphocytes
What is a pathogen?
A pathogen is a disease causing microbe/ microorganism
Non- Specific Defence Mechanisms
Natural Defence barriers
Most microorganisms find it difficult to get inside the body. There are four main ways in which the body prevents potential pathogens from entering.
1.Physical Defence
This skin provides a physical barrier to the entry of pathogens. The tough outer layer of dead cells contains keratin and very little water, which microorganisms need for growth. The skins also secretes various chemicals which inhibit the growth of bacteria such as:
•tears - the lachrymal glands secrete tears which dilute and wash away microorganisms
•sebum - secreted by the sebaceous glands and contains fatty acids that have antimicrobial action
•mucus - a sticky secretion produced by goblet cells that line the air passages, the mucus traps many airborne pathogens
- Mechanical Defence
Nasal hair filter the air that is drawn into the nasal passages. Bacteria and other particles trapped in the mucus are swept away from the lungs by the cilia.
Cilia are tony hair that beat with a wave-like motion. - Chemical Defence
Tears, mucus, saliva and sweat all contain chemicals that inhibit the growth of microorganisms.
Lysozyme is an enzyme found in many of these secretions. It catalyses the hydrolysis of molecules in the cells walls of bacteria.
In addition to lysozyme, sweat contains lactate which also slows bacterial growth.
Hydrochloride acid present in gastric juices kills almost all microorganisms that get as far as the stomach.
The vagina contains harmless bacteria that convert carbohydrate to lactate, which kills pathogenic bacteria. - Biological defence
There are natural populations of harmless bacteria living on the skin and mucous membranes that inhibit the growth of many pathogenic microbes.
They protect us by competing with pathogenic bacteria for nutrients.
Wide-spectrum antibiotics can destroy these useful bacteria and so remove some of the body’s defence.
*The natural defence barrier is the first line of defence, if the microorganisms breach this the second line of defence occurs;
Diagram of Phagocytosis
The Immune Repsonse
The immune response is another aspect of our body’s defence against pathogen- this time is involves the B and T lymphocytes. The immune response depends on the concept of ‘Self and Non-Self’ and on the recognition of antigens. Antigens act as biological markers/ labels and they provide a way in which the body can recognise something that should not be present and therefore may be harmful.
What is an Antigen?
• A chemical capable of provoking the production of specific complementary antibodies - KNOW
Antigens are usually chemicals, particularly proteins or carbohydrates, found on the surface of a cell. When they enter the body, the antigen is recognised by the immune system and trigger an immune response from lymphocytes.
Self and Non-Self
All organisms are genetically different (except for clones and twins) and have different molecules on the surface of their cells. Each different molecule has a very different shape which is determined genetically.
Due to molecules being specific to each individual, the body can distinguish between its own molecules (self) and different molecules (non-self). Normally the body does not respond to ‘self’ cells. However, ‘non self’ cells act as antigens. The body recognises the ‘non self’ cells and this triggers an immune response which will usually destroy the cells carrying antigens.
This reaction to ‘non self’ is one of the main problems in transplanting organs or tissues from one individual to another. The recipient is likely to ‘reject’ to donated organ.
How do Lymphocytes know what is Self and Non-Self?
During foetal development, lymphocytes make contact with foetal ‘self’ cells and are switched off if they are complementary. The remaining ‘switched on’ lymphocytes are those that are not complementary to the ‘self’ cells.
Self and Non-Self Diagram
Lymphocytes
The body contains thousands of different B and T lymphocytes. Each has specific protein receptor sites on its cell surface membrane whose molecular shape in complementary to the shape of one particular antigen. The shapes match by a ‘lock and key’ mechanism. This allows the recognition of and response to a huge range of different antigens.
Summary
B lymphocytes:
•Formed in stem cells within bone marrow
•Development within the bone marrow
•Provide antibody mediated immunity
•Antibodies are produced which responds to antigens found in the body fluids. Usually bacterial and viral infections
T lymphocytes
•Formed in stem cells within bone marrow
•Developed within the thymus gland
•Provides cell mediated immunity
•Responds to antigens attached to body cells. Usually body cells affected by viral infections
How do lymphocytes become sensitised or activated?
When a complementary antigen makes contact with receptors on the surface of a lymphocyte it will become sensitised.
-After being sensitised, the B lymphocyte activates certain genes that set in motion the process which leads to the production of antibodies
-After being sensitised, the T lymphocytes produces different types of T- cells
Antibody-mediated Immunity
- B lymphocytes
Antibody-mediated immunity combats ‘free’ pathogens in blood or tissue fluid. The antibody-mediated immunity is initiated by an activation phase. This is where macrophages (a type of phagocyte) engulf and digest microbes through phagocytosis. This is non specific immunity because the macrophages engulf all foreign cells without specificity. Some of the digested antigens are then displayed on the surfaces of the macrophages as a way of alerting B lymphocytes to act. This display provides other cells of the immune system with an opportunity to recognise the invader and become activated. This is called antigen presentation.
While antigen presentation is taking place B lymphocytes are circulating throughout the bloodstream. Some of the B cells will have cell surface receptors which fits the antigens being presented on the surface of the macrophages. These cells then become activated and respong by dividing repeatedly by mitosis to produce clones of itself, and therefore forming huge numbers of identical B cells over a period of a few weeks.
Note: The period of time between contact with the antigen and the cloning of the B lymphocytes is when the person will suffer the symptoms of the disease.
Some of these activated B cells become plasma cells which produce antibody molecules very quickly. Plasma cells secrete antibodies into the blood, lymph or onto the lining of the lungs and gut. These cells do not live longer than a few weeks before they are destroyed, although the antibodies that they produce can stay in the body for longer.
Other B cells become memory cells that remains circulating in the blood for a long time. If the same antigen is reintroduced a few weeks or months after the first infection, these memory cells divide rapidly and develop into plasma cells and more memory cells. This is respected on every subsequent invasion by the same antigen, which means the infection can be destroyed before the symptoms develop. I.e. the person has become immune to this specific disease.
Antibody-Mediated Immunity
- B lymphocytes
Diagram
Antibodies
Antibodies form a crucial part of the body’s defence against disease.
Antibodies are specific; different antibodies are produced to deal with different pathogens.
Definition of Antibody:
Globular proteins which are specific and complimentary to particular antigens and which can feast to antigens leading the their destruction.
Note: Antibodies being globular structures is good as small changes in the primary structures will result in different globular shapes to allow millions of different antibodies
Action of Antibodies in forming an Antigen-Antibody Complex
Antigen-Antibody Reaction
The Action of Antibodies
•Agglutination- complementary antibodies attach to antigens clumping the bacteria together. Typically, the build up of antibodies in the body fluid will enable a sufficient number to be present to immobilise the bacteria causing their affluent clumping as an antigen-antibody complex. In due course the antigen-antibody complex (clump of bacteria and antibodies) is engulfed by polymorphs and other phagocytes.
•Precipitation- some antibodies bind together soluble antigens into large units which are then precipitated out of solution. As such they are more easily infected by phagocytes (I.e. the antibodies act as opsonins by attaching to pathogens and marking them for phagocytosis)
•Antibodies can also destroy invading cells directly.
Note: Opsonins are molecules which enhance phagocytosis
Cell- Mediated Immunity
-T lymphocytes
In an immune response, the production of T lymphocytes is stimulated by the body’s own cells that have been changed due to the presence of non-self material within them. These cells are referred to as antigen-presenting cells. Examples include:
•Macrophages that have engulfed and broken down pathogens, to then present some of the pathogenic antigens on their own cell surface
•Any type of body that has been invaded by a virus of which antigen are presented on the cell surface membrane of the body cell
•Cancer cells (tumours) may present abnormal antigens on their cell surface membrane
•T lymphocytes will also be activated by transplants, due to the fact that the donated tissue comes from a genetically different individual and will have proteins on its cell surface which the recipients body will recognise as non-self and therefore attack
Antigen-Presenting Cells Bringing about a Response
-Diagram
The Resultant Cloned T-cells
The cloned T cells produced from this response can develop into:
•Killer T-cells (Tc)- Directly destroy infected cells by attaching to the antigens on the cell surface membrane. Killer T-cells can destroy cells by releasing toxic substances such as hydrogen peroxide or nitric acid, or through a protein perforin which creates holes in the cell membrane causing lysis
•Helper T-cells (Th)- When T helper cells are activated they release hormone-like cytokines that stimulate appropriate B cells to divide, develop into plasma cells and secrete antibodies. They also release and attach opsonins to pathogens which marks them to get the attention of phagocytes. They secrete proteins (interferon) which helps limit the ability of viruses to replicate
•Suppressor T-cells (Ts)- Suppresses the immune response of other immune cells. They “switch off” immune responses after invading microbes and infected cells have been destroyed. Also prevents autoimmune response (when the body attack ‘self’ cells in the body)
•Memory T-cells (Tm)-Remain in the body fluid and respond quickly to future infections by the same pathogen
Summary
Summary of Immune Response Involving B and T cells
Summary of the Types of Immunity
Comparison of Different Types of Immunity
Monoclonal Antibody Production
Which Antigen do the Blood Groups Contain?
Which Antibody do the Blood Groups Contain?
Note on Blood Groups Antigen- Antibody content
The ABO Blood Groups Table
Blood Transfusion Table
How Blood Groups are Tested
ELISA
Definitions
Bio Fact Sheet
Bio Fact Sheet (next page)
Lymphocytes Fact Sheet
Lymphocytes Fact Sheet (next page)
The Immune System Overview
