Topic 6: Immunity, Infection and Forensics
Immune Response
Pathogens
A pathogen is any organism which causes disease. Infectious diseases are always caused by
pathogens, which can spread from person to person to infect others. Non-infectious diseases, such
as lung cancer or diabetes, are not caused by pathogens and tend to be more influenced by genetics
or lifestyle factors.
Pathogens include bacteria, viruses, protoctists and fungi. Here are some examples of pathogenic
organisms from each of these kingdoms:
• Bacteria - Mycobacterium tuberculosis causes TB
• Virus - HIV causes AIDS, COVID-19 causes coronavirus, the influenza virus causes seasonal flu
• Protoctist - Plasmodium causes malaria
• Fungi - Trichophyton causes Athlete’s foot
Bacterial cell structure
Bacteria are single-celled prokaryotic organisms
which contain the following structures:
• Circular chromosomal DNA – floats free
in the cytoplasm.
• Ribosomes – these are smaller than
those found in eukaryotic cells (70S)
• Cell wall - made of murein (a glycoprotein)
• Pili - pili are hair-like structures which stick
out from the plasma membrane. They are
used to communicate with other cells
(including the transfer of plasmids between
bacteria).
• Mesosomes - the mesosome is a folded portion of the inner membrane. While some
scientists believe that it plays a role in chemical reactions, such as respiration, other scientists
doubt whether it even exists and think that it may just be an artefact produced during the
preparation of bacterial samples for microscopy.
, • Plasmids - plasmids are small, circular rings of DNA which are separate from the main
chromosome. They house genes which are not crucial for survival but might prove useful -
such as antibiotic-resistance genes, for example. Plasmids can replicate independently from
the main chromosomal DNA.
• Slime capsule - in addition to a cell wall, some bacteria also have a capsule which is made of
slime. The main function of the capsule is to protect the bacterium against an immune system
attack.
• Flagellum – rotates to move the bacterium
HIV and AIDS
Human immunodeficiency virus (HIV) is a deadly virus which can weaken a person’s immune
system by destroying a type of immune cell called T helper cells. It is spread through the
transmission of infected bodily fluids (e.g. during sexual intercourse, sharing of needles or blood
transfusion). HIV doesn’t kill the patient directly, but weakens the immune system to an extent that the
patient is unable to fight off other infections which normally would not pose a threat - these are known
as opportunistic infections.
When the patient’s immune system has weakened to a particularly low level (this is seen by a low T
helper cell count in hospital blood tests), the patient has developed a disease called acquired
immunodeficiency syndrome (AIDS). The time between the initial infection and the onset of AIDS
symptoms (the latency period) varies greatly between individuals but is usually around 10 years. The
length of the latency period depends on the individual’s age, strength of their immune system and
access to healthcare.
The symptoms of AIDS follow a series of stages:
1. The person will first develop less serious, minor infections which will gradually become more and
more severe.
2. As the patient’s body tries to fight off more infections, the lower their T cell levels drop.
3. Eventually their immune system will be so weak that they will die of an opportunistic infection
because they do not have enough immune cells to defend themselves.
Immune Response
Pathogens
A pathogen is any organism which causes disease. Infectious diseases are always caused by
pathogens, which can spread from person to person to infect others. Non-infectious diseases, such
as lung cancer or diabetes, are not caused by pathogens and tend to be more influenced by genetics
or lifestyle factors.
Pathogens include bacteria, viruses, protoctists and fungi. Here are some examples of pathogenic
organisms from each of these kingdoms:
• Bacteria - Mycobacterium tuberculosis causes TB
• Virus - HIV causes AIDS, COVID-19 causes coronavirus, the influenza virus causes seasonal flu
• Protoctist - Plasmodium causes malaria
• Fungi - Trichophyton causes Athlete’s foot
Bacterial cell structure
Bacteria are single-celled prokaryotic organisms
which contain the following structures:
• Circular chromosomal DNA – floats free
in the cytoplasm.
• Ribosomes – these are smaller than
those found in eukaryotic cells (70S)
• Cell wall - made of murein (a glycoprotein)
• Pili - pili are hair-like structures which stick
out from the plasma membrane. They are
used to communicate with other cells
(including the transfer of plasmids between
bacteria).
• Mesosomes - the mesosome is a folded portion of the inner membrane. While some
scientists believe that it plays a role in chemical reactions, such as respiration, other scientists
doubt whether it even exists and think that it may just be an artefact produced during the
preparation of bacterial samples for microscopy.
, • Plasmids - plasmids are small, circular rings of DNA which are separate from the main
chromosome. They house genes which are not crucial for survival but might prove useful -
such as antibiotic-resistance genes, for example. Plasmids can replicate independently from
the main chromosomal DNA.
• Slime capsule - in addition to a cell wall, some bacteria also have a capsule which is made of
slime. The main function of the capsule is to protect the bacterium against an immune system
attack.
• Flagellum – rotates to move the bacterium
HIV and AIDS
Human immunodeficiency virus (HIV) is a deadly virus which can weaken a person’s immune
system by destroying a type of immune cell called T helper cells. It is spread through the
transmission of infected bodily fluids (e.g. during sexual intercourse, sharing of needles or blood
transfusion). HIV doesn’t kill the patient directly, but weakens the immune system to an extent that the
patient is unable to fight off other infections which normally would not pose a threat - these are known
as opportunistic infections.
When the patient’s immune system has weakened to a particularly low level (this is seen by a low T
helper cell count in hospital blood tests), the patient has developed a disease called acquired
immunodeficiency syndrome (AIDS). The time between the initial infection and the onset of AIDS
symptoms (the latency period) varies greatly between individuals but is usually around 10 years. The
length of the latency period depends on the individual’s age, strength of their immune system and
access to healthcare.
The symptoms of AIDS follow a series of stages:
1. The person will first develop less serious, minor infections which will gradually become more and
more severe.
2. As the patient’s body tries to fight off more infections, the lower their T cell levels drop.
3. Eventually their immune system will be so weak that they will die of an opportunistic infection
because they do not have enough immune cells to defend themselves.
