ALLERGIES AND THE DEFENCE AGAINST DISEASES #2

INTRODUCTION

A pleasant greeting to all my beloved readers. A big thanks for the encouragement and support.

In my last post titled ALLERGIES AND THE DEFENCE AGAINST DISEASES, I discussed allergies and introduced immunity as a defense against diseases among other subjects. Still under immunity, I explained the non-specific immune response such as inflammation etc. But, today, I'll be shedding more light on the other part of immunity which is the specific immunity and also on vaccinations and blood transfusions.

Pixabay

SPECIFIC IMMUNITY

The specific immune system protects the body from ‘invasion’ by microorganisms and parasites and also makes sure that the body’s defences 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

When a B cell encounters its triggering antigen, it gives rise to many large cells known as plasma cells.

[Adobe Illustrator. • Public domain]

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 uses 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 site of infection. B cells do not fight pathogens directly.

An antibody, or Immunoglobulin, 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 (we say they are 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. This sequence of events is described as a primary immune response.

So that the body can respond more quickly next time, 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 antibody. This response is called a secondary immune response and is very much quicker and more effective than the primary response.

This 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 becoming 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.

Lymphocyte B cell

[Blausen Medical • CC BY-SA 4.0]

VACCINATIONS

Several infectious diseases overwhelm the normal primary immune response and so can be fatal on first exposure. Thankfully, the specific immune response was speeded up 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 made less virulent − less able to produce disease. Examples of this are shown in the table below.

Recommended vaccination schedule

Modern vaccines are usually very safe but they can cause local swelling around the injection site, and a feeling of being unwell for a couple of days, caused by the reaction they set up in the body. A very small proportion of people getting a vaccine will have a bad reaction to it, probably caused by a rare allergy, a drug interaction, or some other factor. For this one-in-a-million person, the dangers of vaccination are very real, but for the population generally, the benefits outweigh the dangers.

HOW MOTHERS GIVE IMMUNITY TO THEIR BABIES

When babies are born, they emerge from the protective environment of their mother’s 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 pass 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 it can help a baby to fight off common pathogens.

[Pixabay]

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 allergens 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 suppress mast cells or neutralise histamine – chemists sell many antihistamines in the pollen season.

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 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 (e.g. parasites) directly. The 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 are 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 was no need, this could damage the body and would be, at best, a waste of resources.

PRESENTING ANTIGENS

As I wrote in my previous post, bacteria that are engulfed by phagocytic white cells are fragmented and their antigens are expressed on the surface of the infected cell in combination with proteins from the MHC complex. This is a vital step in setting up a cell-mediated immune response. T helper cells are activated when they come into contact with an antigen-presenting cell and they are then specific for the antigen that has been presented to them.

Activated and specific T helper cells then produce chemicals called cytokines that circulate in the blood, activating B cells and also T killer cells. The cytokines also stimulate the specific B cells and T killer cells to divide, forming large clones of active immune cells. The T killer cells are specific for the presented antigens and so recognise and kill only body cells that are expressing these antigens in combination with MHC proteins. This ensures that only infected cells are destroyed. The T killer cells attach themselves to cells expressing MHC-linked antigens and then release chemicals that cause the infected cell to lyse (split open).

BLOOD TRANSFUSIONS AND ORGAN TRANSPLANTS

In the past 100 years, advances in modern medicine have led to the technology and knowledge that allow doctors to give one person’s cells, tissues and organs to another person. One of the main objectives in developing these life-saving treatments has been to overcome the natural reaction of the immune system to destroy transplanted cells and tissue, which it ‘sees’ as non-self.

BLOOD GROUPING AND TRANSFUSIONS

Towards the end of the nineteenth century, medical scientists realised that accidents were often fatal simply due to blood loss. Losing large volumes of blood sent victims into shock and they died, even though their injuries were otherwise not too severe. Many women, in paricular, often died when they lost blood when giving birth. Different doctors tried transfusing blood from healthy person into the injured person, to try to restore their blood volume.

Sometimes this worked and sometimes it didn’t. When it failed, the results were disastrous. We now know that if the blood groups of the donor (the person giving the blood) and the recipient (the person receiving it) are different, the recipient’s immune system reacts against the donated blood, producing a massive and deadly immune response.

In the early 1900s, an Austrian scientist, Karl Landsteiner, discovered that red blood cells from different people had different sets of antigens on their surface. The entire human population can be placed into one of four main blood groups: A, B, AB and O, according to the antigens on their red blood cells.

People with blood group O have no antigens on the surface of their red blood cells. Blood from these people cannot cause an adverse immune response if it is transfused into people from the other three blood groups. We say that people with blood group O are universal donors. By the same reasoning, people with blood group AB cannot have any antibodies to either of the blood group antigens and so cannot mount an immune response if they are given blood by people in any of the other three groups. We say that people with AB blood are universal recipients.

Blood transfusions are dangerous when the recipient has antibodies in their blood that react with antigens present on the surface of red cells in the donor blood. So, for example, a person of blood group A cannot donate blood to someone with group B. The recipient in this case has anti-A antibodies which react with the A antigens on the red blood cells of the donor blood.

So, this table below shows us about which transfusions are possible.

Diagram of ABO blood groups and the IgM antibodies present in each [InvictaHOG • Public domain]

Thanks for reading.

Vaccination - Wikipedia