The immune system

The Immune System: 

•  We are able to overcome pathogens (disease causing organisms) because we have an immune system.
•  This is a complex system involving many different cells and tissues that allows us to develop immunity – resistance to infections.

Common pathogens include bacteria, fungi, viruses and protoctists. This last group includes microscopic parasites.

A pathogenic organism is able to;

• Break through the physical barriers of the body and enter tissues or cells,
• Resist the efforts of the immune system to destroy it, long enough to multiply inside the host’s body,
• Get out of one host and into another,
• Damage the hosts tissues – either directly, or indirectly by means of toxins (poisons) that it releases

The non-specific immune response;

When the body is damaged by cuts, scratches or burns, or is attacked by a pathogenic organism that manages to breach it defences, it produces a non-specific immune response. It is called a non-specific response because it occurs in response to tissue damage itself, not to the cause of the damage.

Inflammation:

Inflammation is a rapid response to tissue damage. Whether it is in response to a cut, insect bite or a heavy blow such as a sport injury, the classic signs of inflammation are the same:

• Redness ->  blood vessels dilate, increasing bloody flow to the area.
• Heat -> also caused by the extra blood flow.
• Swelling -> extra blood forces more tissue fluid into damaged tissues.
• Pain -> swollen tissues press on receptors and nerves. Also chemicals produced by cells in the area stimulate the nerves. 

Inflammation

Inflammation is triggered by damaged cells. Ruptured cells and white cells release ‘alarm’ chemicals such as histamine.

These substances dilate blood vessels and increased blood flow leads to the classic signs of inflammation. The ‘alarm’ chemicals also attract white cells that remove bacteria and debris by phagocytosis.

Why is inflammation useful?

• Inflammation prevents the spread of infection and speeds up the healing process.
• It also provides a way of telling the rest of the immune system what is going on. When microorganisms are phagocytosed, fragments of their cells are processed by phagocytes. Some of these surface molecules (antigens) allow the specific immune system to recognise and remember the type of microorganism that has tried to invade the body.
• Antigens stimulate the specific immune system to produce cells and chemicals that bind specifically to that antigen, and to no others. 

Phagocytosis:

Neutrophils are the commonest type of white cell. Together with monocytes they are known as phagocytes because of their ability to ‘eat’ pathogens by phagocytosis.

Phagocytosis involves the following steps;

• The membrane of the neutrophil extends and surrounds the bacterium.
• The bacterium ends up inside the cell cytoplasm, contained within a vacuole.
• Lysosomes that contain digestive enzymes and free radicals fuse with the vacuole that has the bacterium inside.
• The bacterium is digested into fragments within the vacuole.
• When the vacuole fuses with the cell membrane, bacterial fragments bound to proteins become part of the cell membrane and stick out from the surface of the neutrophil.
• The antigens on the bacterium are presented on the outside of the neutrophil. They can now be seen well by other cells of the immune system, enhancing the immune response against the bacteria. 

Specific immunity:

The specific immune system protects the body from ‘invasion’ by microorganisms and parasites and also makes sure that the body’s defence do not turn on its own tissues. The specific immune response is made up of two different systems that co-operate closely.

Humoral immunity: also called antibody-mediated immunity involves only chemicals, no cells are directly involved. The chemicals, called antibodies, attack bacteria and viruses before they get inside body cells. They also react with toxins and other soluble foreign proteins. Antibodies are produced by white cells called B lymphocytes, or B cells.

Cell mediated immunity, as the name suggests, involves cells that attack foreign organisms directly. Activated T lymphocytes, or T cells kill some microorganisms, but they mostly attack infected body cells. The body used cell-mediated immunity to deal with multicellular parasites, fungi, cancer cells and rather unhelpfully, tissue transplants. 

B cells and humoral immunity;

• B cells are produced in the bone marrow and are distributed throughout the body in the lymph nodes. B cells respond to the foreign antigens of a pathogen by producing specific antibodies. Antibodies are complex proteins that are released into the blood and carried to the side of infection. B cells do not fight pathogens directly.
• An antibody, or immunoglobin, is a Y-shaped protein molecule that is made by a B lymphocyte in response to a particular antigen. Antibodies interact with the antigen and render it harmless.
• When a pathogen tries to invade the body for the first time, each of its antigens activates one B cell, which divides rapidly to produce a large population of cells. All the new cells are identical (clones) and they all secrete antibodies specific for the invading pathogen. When the infection is over, most of the newly made B cells die: their job is done. The primary immune response.

B cells and humoral immunity; 

• So that the body can respond more quickly next time round, some of the activated B cells persist in the body for several years. These memory cells ‘remember’ what the pathogen is like and, if it tries to invade again, they divide rapidly to produce an even greater number of active B cells, all capable of secreting specific antibodies. This response is called the secondary immune response, and is very much quicker and more effective than the primary response.
• The ability of the immune system is central to vaccination a vaccine stimulates the body to produce a primary immune response to a particular pathogen, without being infected by it. A subsequent booster produces a secondary response. Later if the pathogen tries to invade, the body can mount a very fast response and the person does not become ill. 

T cells and cell mediated immunity:

Like B cells, T cells respond to specific antigens. When a pathogen first infects the body, each individual antigen stimulates a single T cell. This divides to a form a clone, in the same way that B cells do. Some of the activated T cells become memory cells and persist in the body, ready to mount a secondary response if the pathogen attacks again. The others however do not produce antibodies. They develop further to become one of three types of T cells:

• T helper cells: are so called because they help with, or rather control, the rest of the specific immune response. They cause B cells to divide and then to produce antibodies, they activate the two other sorts of T cell and they activate macrophages, so the macrophages are ready to phagocytose pathogens and debris.
• T killer cells:  attack infected body cells and the cells of some larger pathogens directly. They two cells face each other, membrane to membrane, and the T killer cell punches holes in its opponent. The infected cell or parasite loses cytoplasm and dies.
• T suppressor cells: Is a sort of safety cut-out mechanism. When the immune response becomes excessive, or when the infection has been dealt with successfully, these T cells damp down the immune response. This is a good idea: if the body continued to make antibodies and stimulate more and more T and B cells to divide, even when there is no need, this could damage the body and would be, at best, a waste of resources. 

Vaccinations;

• Several infectious diseases overwhelm the primary immune response and so can be fatal on first exposure. Thankfully we are able to speed up the specific immune response by giving vaccines against the pathogens that cause them.
• The basic idea behind a vaccine is that it contains some form of the pathogen, so that it stimulates memory cells to develop, ready to destroy the real pathogen should it be encountered.
• Obviously the vaccine can’t simply be the pathogen itself, or the toxins it makes. Somehow the vaccine must be less virulent – less able to produce disease. 

How mothers give immunity to their babies:

• When babies are born, they emerge from the protective environment of their mothers uterus. They are exposed to many potential pathogens. Healthy babies have a fully functional immune system and can mount primary immune responses to many different antigens straight away. However, babies also get a bit of extra help from their mothers.
• Antibodies *** across the placenta and, after birth, the supply continues through breast milk. Babies have very porous intestines that can absorb these large proteins directly into the bloodstream without digesting them. These large pores close by the age of one year.  We call this kind of immunity – passed from one person to another – passive immunity. It does not last long, because the antibodies are broken down within a few days, but I can help a baby to fight off common pathogens.
• Passive immunity is also used to treat some types of poisoning, such as snakebites. Antiserum, blood that contains antibodies specific to a particular snake venom, is produced in horses, purified, and then given to people who have been bitten by a snake. 

Allergies:

• An allergy or hypersensitivity is reaction to the presence of a normally harmless substance called an allergen. Some common allergies are pollen, house dust mites, animal fur and feathers, fungal spores, insect bites and penicillin.
• The commonest symptoms of an allergy are sore eyes, runny nose, sneezing and asthma. Many of these symptoms result from inflammation of the mucous membranes, caused by mast cells, which release chemicals such as histamine. Many anti-allergy treatments supress mast cells or neutralise histamine – chemists sell many antihistamines in the pollen seasons. 

Summary:

• We are surrounded by potential pathogens such as bacteria and viruses which can cause disease if allowed to enter and multiply inside the body.
• The body’s defence systems consist of barriers to keep microorganisms out, and mechanisms to detect and destroy those that do enter. Together, these mechanisms are the immune system.
• The immune system is capable of non-specific responses and specific responses. Non-specific mechanisms (inflammation and phagocytosis) occur in response to any invading microorganism. Specific mechanisms allow the body to recognise and fight individual types of microorganism. There are two specific response; cell-mediated immunity and humoral immunity.
• Lymphocytes are responsible for the specific immune response. T cells mature in the thymus gland and B cells come directly from bone marrow. Both are able to recognise foreign antigens that come from pathogens.

Summary: 

• B cells are responsible for humoral immunity: they secrete specific proteins called antibodies.
• T cells are responsible for cell-mediated immunity and help control the overall specific response. T killer cells attack pathogens or infected cells. T helper cells activate B cells, telling them to secrete antibodies. T suppressor cells damp down the immune system when the infection is over.
• Autoimmune diseases occur when the body’s immune system attacks its own tissues. Examples are, rheumatoid arthritis, multiple sclerosis and myasthenia gravis. Allergies occur when the body ‘over-reacts’ to a normally harmless substance such as pollen.
• When cells or tissues are taken from one individual and given to another, they may be recognised as foreign and destroyed. This is often called graft-rejection. To minimise the chance of rejection in blood transfusions and organ transplants it is important to match blood and tissue types.
• Antibiotics are important drugs for treating infections caused by bacteria. In the time since antibiotics were first developed, bacteria have evolved to become resistant to them. Antibiotic resistant strains of many bacteria are becoming common and represent a problem for many doctors and researchers.