What is HIV?
Graphic used with permission of howstuffworks.com
The Human Immune System
- In order to understand HIV, one must understand the human immune system. The first line of defense is a person’s skin, mucous
membranes, and other secretions which prevent pathogens from ever entering your body. Pathogens are considered things your body
does not want, for example bacteria and viruses.
- The second line of defense includes nonspecific mechanisms which attempt to contain the spread of pathogens throughout one’s
body. The second line of defense relies heavily on the use of white blood cells, which ingest invading organisms. About 5% of white blood
cells are made of monocytes, which develop into macrophages. The role of these macrophages is vital to the human immune system, as they
are able to engulf pathogens without having to self destruct.
- The body’s third line of defense is a highly specific means of distinguishing “self” from “non-self” and destroying all “non-self”. All of
one person’s cells are marked with a unique set of proteins which label them as “self”. Certain cells in the body are capable of recognizing
every antigen (molecules belonging to viruses/bacteria) that may enter one’s body over a lifetime.
These cells include macrophages, T-Cells, B Cells, and interior thymus cells. These cells rely on Helper T-Cells to alert them of antigens in
the body, thus creating an immune response. Once recognized, Killer T-Cells actively destroy pathogens and even the body’s own cells if
that have been invaded by a pathogen.
How HIV attacks the Body
- As commonly known, HIV cannot penetrate your immune systems first line of defense. You cannot contract HIV by breathing bad air
or by holding the hand of somebody who is HIV positive. You have to work hard to become infected by doing things such as sharing
contaminated needles, or by having unprotected sex with an infected person. Unfortunately, infected mothers can also transmit the virus
to their unborn children or by means of breast milk. Basically, once HIV is in your system it is already to your third and final line of defense.
- HIV doesn’t target just any cell, it goes right for the cells that want to kill it, i.e. the monocytes, macrophages, and Helper T-Cells. Once
HIV infects these cells, T-Cells come along and destroy those infected cells, thus one’s own body is killing off the mechanisms needed to
destroy the virus. The virus can infect 10 billion cells a day, yet only 1.8 billion can be replaced daily. Thus, after many years of a constant
battle, the body has insufficient numbers of T-Cell to mount an immune response against infections. At the point when the body is unable
to fight off infections, a person is said to have the disease AIDS. This means that it is not the virus or the disease that ultimately kills a
person; it is the inability to fight of something as minor as the common cold.
How does HIV get into cells and infect them?
- Helper T-Cells bind to antigen presenting cells (APC’s) by means of a receptor on the cell surface called CD4. HIV is able to use it’s
own gp120 (a protein on the surface of HIV) to bind to a cells CD4. HIV also binds to coreceptors CCR5 and CXCR4 of the cell surface.
HIV’s membrane fuses to the cell membrane and gains entry into the cell. HIV is one of the few retroviruses, meaning that it can convert
its two strands of RNA into DNA by use of the enzyme reverse transcriptase. Because it has two copies of its RNA, it has two chances to
in case one of the strands does not work properly or is damaged. The virus then permanently integrates the newly formed DNA into the
host’s genome. This means the cells is able to release more HIV into your body, and the process continues.
The University of Arizona
Biology 181 Honors
Last Updated: December 5, 2002
All contents copyright (c) 2002. All rights reserved.