Infection +Responce Flashcards
What is immunity
Sometimes the body’s defence mechanisms overwhelm the pathogen and the individual recovers from the disease. Having overwhelmed the pathogen, however, the body’s defences seem to be better prepared for a second infection from the same pathogen and can kill it before it can cause any harm. This is known as immunity and is the main reason why some people are unaffected by certain pathogens.
Defence mechanisms
The human body has a range of defences to protect itself from pathogens). Some are general and immediate defences .Others are more specific, less rapid but longer-lasting. These responses involve a type of white blood cell called a lymphocyte and take two forms:
• cell-mediated responses involving T lymphocytes
• humoral responses involving B lymphocytes.
Recognising your own cells
To defend the body the lymphocytes must be able to distinguish between body cells and forge in material to prevent the lymphocytes from damaging bodily tissues ,each cell contains moles on the surface that enables detection from lymphocytes the most important molecule on the surface are the proteins , this is due to the enormous highly specific tertiary 3d structure which enables the lymphocytes to decider between body cells ,pathogens and forge in cells
Now lymphocytes recognise cells
• There are probably around ten million different lymphocytes present at any time, each capable of recognising a different chemical shape.
• In the fetus, these lymphocytes are constantly colliding with other cells.
• Infection in the fetus is rare because it is protected from the outside world by the mother and, in particular, the placenta.
• Lymphocytes will therefore collide almost exclusively with the body’s own material (self).
• Some of the lymphocytes will have receptors that exactly fit those of the body’s own cells.
• These lymphocytes either die or are suppressed.
• The only remaining lymphocytes are those that might fit foreign material (non-self), and therefore only respond to foreign material.
• In adults, lymphocytes produced in the bone marrow initially only encounter self-antigens.
• Any lymphocytes that show an immune response to these self-antigens undergo programmed cell death (apoptosis) before they can differentiate into mature lymphocytes.
• No clones of these anti-self lymphocytes will appear in the blood, leaving only those that might respond to non-self antigens.
What are phagocytes
Large particles, such as some types of bacteria, can be engulfed by cells in the vesicles formed from the cell-surface membrane. This process is called phagocytosis. In the blood, the types of white blood cells that carry out phagocytosis are known as phagocytes. They provide an important defence against the pathogens that manage to enter the body. Some phagocytes travel in the blood but can move out of blood vessels into other tissues.
Phagocytosis
• Chemical products of pathogens or dead, damaged cells act as attractants, causing phagocytes to move towards the pathogen (e.g., a bacterium).
• Phagocytes have several receptors on their cell-surface membrane that recognise, and attach to, chemicals on the surface of the pathogen.
• They engulf the pathogen to form a vesicle, known as a phagosome.
• Lysosomes move towards the vesicle and fuse with it.
• Enzymes called lysozymes are present within the lysosome. These lysozymes destroy ingested bacteria by hydrolysis of their cell walls.
Antigens
An antigen is any part of an organism or substance that is recognised as non-self (foreign) by the immune system and stimulates an immune response. Antigens are usually proteins that are part of the cell-surface membranes or cell walls of invading cells, such as microorganisms, or abnormal body cells, such as cancer cells. The presence of an antigen triggers the production of an antibody as part of the body’s defence system
Lymphocytes
Immune responses such as phagocytosis are non-specific) and occur whatever the infection. The body also has specific responses that react to specific antigens. These are slower in action at first, but they can provide long-term immunity. This specific immune response depends on a type of white blood cell called a lymphocyte.
B lymphocytes (B cells) are so called because they mature in the bone marrow. They are associated with humoral immunity, that is, immunity involving antibodies that are present in body fluids, or
‘humour’ such as blood plasma. This is described in more detail in Topic 5.5.
• T lymphocytes (T cells) are so called because they mature in the thymus gland. They are associated with cell-mediated immunity, that is immunity involving body cells.
Cell mediated immunity
T lymphocytes can distinguish these invader cells from normal cells because:
• phagocytes that have engulfed and hydrolysed a pathogen present some of a pathogen’s antigens on their own cell-surface membrane
• body cells invaded by a virus present some of the viral antigens on their own cell-surface membrane
. transplanted cells from individuals of the same species have different antigens on their cell-surface membrane
• cancer cells are different from normal body cells and present antigens on their cell-surface membranes.
Stages to lymphocytes to infection
Pathogens invade body cells or are taken in by phagocytes.
2 The phagocyte places antigens from the pathogen on its cell-surface membrane.
3
Receptors on a specific helper T cell (T, cell) fit exactly onto these antigens.
4
This attachment activates the T cell to divide rapidly by mitosis and form a clone of genetically identical cells.
5 The cloned T cells:
a develop into memory cells that enable a rapid response to
future infections by the same pathogen
b stimulate phagocytes to engulf pathogens by phagocytosis
c stimulate B cells to divide and secrete their antibody
d activate cytotoxic T cells (Tc cells).
Now cytoxic T cells kill infected cells
Cytotoxic T cells (T, cells) kill abnormal cells and body cells that are infected by pathogens, by producing a protein called perforin that makes holes in the cell-surface membrane. These holes mean the cell membrane becomes freely permeable to all substances and the cell dies as a result.
Monoclonal antibodies
B cells divide into clones , the clones produce an antibodies that is specific to that clone only , the antibodies produced by these clones are called monoclonal antibodies
The process of humoral immunity
The surface antigens of an invading pathogen are taken up by a B cell.
The B cell processes the antigens and presents them on its surface.
Helper T cells attach to the processed antigens on the B cell thereby activating the B cell.
The B cell is now activated to divide by mitosis to give a clone of plasma cells.
The cloned plasma cells produce and secrete the specific antibody that exactly fits the antigen on the pathogen’s surface.
The antibody attaches to antigens on the pathogen and destroys them
Some B cells develop into memory cells. These can respond to future infections by the same pathogen by dividing rapidly and developing into plasma cells that produce antibodies. This is the secondary immune response.
Antibodies
Antibodies are made up of four polypeptide chains. The chains of one pair are long and are called heavy chains, while the chains of the other pair are shorter and are known as light chains. Each antibody has a specific binding site that fits very precisely onto a specific antigen to form what is known as an antigen-antibody complex. The binding site is different on different antibodies and is therefore called the variable region. Each binding site consists of a sequence of amino acids that form a specific 3-D shape that binds directly to a specific antigen. The rest of the antibody is known as the constant region.
This binds to receptors on cells such as B cells.
Ways antibodies realms to destruction os pathogen
• They cause agglutination of the bacterial cells (Figure 2). In this way clumps of bacterial cells are formed, making it easier for the phagocytes to locate them as they are less spread-out within the body.
• They then serve as markers that stimulate phagocytes to engulf the bacterial cells to which they are attached.
