Human immunodefi ciency virus (HIV, Figure 6.14, overleaf ), fi rst
identifi ed in the early 1980s, causes the series of symptoms together known as acquired immune defi ciency syndrome, or AIDS. HIV infects only the helper T-cells, a type of lymphocyte that is important in maintaining communication between cells of the immune system. After a latent period of months or years, helper T-cells are gradually destroyed and, as their
Figure 6.11 Antibody production.
Figure 6.12 The basic structure of an antibody molecule.
light chain
disulfide links
heavy chain polysaccharide chains polypeptides
Variable region of the four polypeptides;
this is the part that binds to the antigen.
numbers fall, so does the body’s ability to fi ght infection. Helper T-cells instruct other lymphocytes to clone and generate antibodies, and without them an infected person can no longer fi ght off pathogens. Secondary infections result and the person is said to be suff ering from AIDS.
Cause, transmission and consequences of AIDS
AIDS is the end stage of an HIV infection. It is caused by a severe failure of the immune system as the HIV virus selectively infects helper T-cells.
Some infected individuals have no symptoms in the early stages of the disease while others may be slightly unwell when fi rst infected. Symptoms of AIDS develop as the number of active helper T-cells decreases. The symptoms occur as a result of secondary infections caused by bacteria, fungi and viruses that the body is unable to resist due to its compromised immune system (Figure 6.15, overleaf ).
HIV is transmitted in blood, vaginal secretions, semen, breast milk and sometimes across the placenta. In some countries, HIV has been transmitted in blood transfusions but in most places with medical care facilities, blood for
Figure 6.14 HIV viruses consist of a spherical glycoprotein and lipid coat enclosing two strands of RNA. The virus is 60 times smaller than a red blood cell.
Figure 6.13 The various ways in which different antibodies can destroy bacteria or their toxins.
capsid reverse
transcriptase RNA
molecule
envelope 120 nm
glycoprotein phospholipid
core antibody
pathogenic bacteria
toxin produced by bacteria
Some antibodies bind and stimulate phagocytes to ingest and kill the pathogen.
Some antibodies neutralise toxins produced by pathogens.
Some antibodies neutralise a pathogen by binding to it.
HIV is a retrovirus, which means it can insert its DNA into that of a host cell using a protein called reverse transcriptase. Even if all the viruses in the body could be removed, the T-cells would continue to make new viruses.
6 HUMAN HEALTH AND PHYSIOLOGY 1 145 transfusion is now screened for the virus. The virus is most frequently passed
from person to person in bodily fl uids during sex and also when non-sterile syringe needles are used to administer either legal or illegal drugs.
AIDS used to be thought of as a disease that aff ected only drug abusers and homosexuals but it is now clear that the virus is also transmitted by heterosexual sex. Transmission is most likely among individuals who engage in casual sex with multiple partners.
AIDS is a worldwide pandemic but some regions are more seriously aff ected than others. In 2008, the number of people living with HIV and AIDS was estimated at 25 million in sub-Sarahan Africa and almost 8 million in South and South East Asia. AIDS is the main cause of death for men and women aged between 16 and 50 years in these countries, Latin America and the Caribbean. Those who die are farmers, workers and parents, so the social and economic consequences are signifi cant – in many communities, the elderly are left caring for children, with little support.
Family incomes and national productivity decline as people of working age fall ill. Food production is aff ected and causes more problems. Caring for the sick and providing medical care form an expensive burden for individual families, companies and governments.
Figure 6.15 HIV infection proceeds through three stages – HIV positive with few symptoms, some symptoms and low helper T-cell count, and clinical AIDS with associated symptoms.
First infection
Virus particles are carried from the body fluid of one person to the body fluid of another.
Multiplication The virus enters a helper T lymphocyte.
A
C B
A DNA copy is made of the viral RNA.
The DNA copy is inserted into a chromosome.
The lymphocyte divides with viral DNA in its chromosome.
The cell remains normal (latent virus).
perhaps many years later … Viral DNA takes
over the cell and more HIV is made.
The cell dies and releases thousands of HIV particles.
0
A HIV positive, with antibodies in blood, but few or no symptoms
B some symptoms, falling helper T-cell count
C clinical AIDS symptoms appear
1 2 3
Years after infection
number of virus particles
helper T-cell count
4 5 6 years later …
10 Defi ne the term ‘pathogen’.
11 Describe what is meant by the term ‘antigen’.
12 State why helper T-cells are important in the immune response.
13 Distinguish between a ‘phagocyte’ and a ‘lymphocyte’.
6.4 Gas exchange
All living cells need energy for their activities. Energy is released from the breakdown of glucose and other substances during the process of cell respiration. Respiration is a chemical reaction that occurs in mitochondria and the cytoplasm and releases energy as ATP, a form that can be used inside cells.
Our cells use oxygen to carry out aerobic respiration and produce carbon dioxide as a waste product. Oxygen is taken in from the air and carbon dioxide is returned to it in a passive process known as gas exchange. Gas exchange occurs in the alveoli of the lungs where oxygen from the air diff uses into blood capillaries, and carbon dioxide passes in the opposite direction. Gases are also exchanged in the tissues where oxygen diff uses into respiring cells and is exchanged for carbon dioxide.
Whenever diff usion occurs, there must always be a concentration gradient with a higher level of the diff using substance in one area than in another. Air inside the alveoli contains a higher concentration of oxygen than the blood, so oxygen diff uses into the blood. Blood contains a higher level of carbon dioxide than inhaled air, so carbon dioxide diff uses into the alveoli.
For gas exchange to continue, these concentration gradients must be maintained. As oxygen diff uses out of the alveoli, the level of oxygen inside them gradually falls. Stale air must be expelled regularly and replaced with a fresh supply. This is achieved by breathing in and out, a process known as ventilation.