GCSE Biology: Immunity and disease

Justina: There are several thousand diseases that affect human beings.

Georgina: And these diseases can be classified into two distinct types: communicable diseases that are usually transferred from one organism to another; and non-communicable diseases, which are not.

Justina: Examples of non-communicable diseases are cancer, diabetes and genetic diseases.

Georgina: But we’re going to take a closer look at communicable diseases which are spread through microorganisms that can cause disease, commonly called pathogens.

Justina: In most cases, a pathogen will infect a host, reproduce or replicate and then spread from their host to other organisms.They can spread through a variety of ways:direct contact, water, air or on food.Many of these pathogens are bacteria.

Georgina: Bacteria are single-celled organisms that don’t have a nucleus.Their DNA floats around in the cell cytoplasm in a long loop or in smaller loops called plasmids.Bacteria often have pili to help them attach to surfaces, or tails called flagella, to help them move.

Justina: Bacteria will infect a host and then begin to rapidly reproduce using a form of asexual reproduction, called binary fission, which is a similar process to mitosis.
Binary fission creates identical genetic daughter cells form the parent cell.

Georgina: The bacteria can then be transferred to other organisms from the host.Bacterial infections are often treated with antibiotics.Antibiotics are chemicals produced by various fungi, that kill types of bacteria by breaking down the cell wall and inhibiting their cellular process, preventing bacteria reproduction.Alexander Fleming discovered the first antibiotic, Penicillin, in the early twentieth century.Many others have been discovered since.

Justina: Other diseases are caused and spread by viruses.Viruses, unlike bacteria, are not considered to be alive.They are made up of a relatively short length of genetic material which is surrounding by a protein coat.Georgina: Most virus particles invade suitable host cells, then replicate themselves within the cell thousands of times, which destroys the host cell.The virus then invades other nearby cells.Some virus particles join their DNA to the DNA in the host cell, creating infected cells that then create clones of themselves.

Justina: Unlike bacterial infections, viruses cannot be treated with antibiotics.Viruses have a protective protein coat and no cell wall.Therefore, antibiotics cannot get inside.

Georgina: The human body has adapted defence mechanisms that help protect it against infection by microorganisms.The first of these is the non-specific defence system.This is the body’s first line of defence and includes the skin, tears, the nose and stomach acid; all of which prevent pathogens from going further into the body and the bloodstream.

Justina: The specific immune system comes into play when pathogens get into the bloodstream.Two types of white blood cells play a very important role.Lymphocytes and phagocytes.

Georgina: Any invading pathogen will have antigens on their surface that the lymphocytes recognise as being from outside the body.

Justina: The lymphocytes recognise the antigens and produce specific antibodies that bind to the antigens on pathogens, causing them to clump together.This makes it easier for phagocytes to do their work.

Georgina: Phagocytes engulf the pathogen.Enzymes released by the phagocyte then break down and destroy the pathogen.

Justina: While the immune system is our body’s natural defence mechanism, we have also developed other medicines and treatments that support the immune system.

Georgina: Vaccines, for example, introduce a dead or inactive form of a pathogen into the body.The lymphocytes then produce specific antibodies and become memory lymphocytes that can create the same antibodies more quickly and in greater number if the antigen is detected on a pathogen in the future.

Justina: It’s amazing what medicine can do.

Georgina: It is, but a strong immune system is vital in fighting pathogens, and medical treatments are there to support and enhance the body’s natural defences.

Justina: Teamwork!

Georgina: Exactly.