CELL-MEDIATED IMMUNITY
Cell-mediated immunity, also known as cellular immunity, uses T-leukocytes
(referred to as natural killer cells or NK cells) to attack non-self cells. Cell-
mediated immunity also causes the body to release cytokines, which regulate the
activities of antibody-mediated immunity and inflammation.
Cell-mediated immunity is especially useful in identifying and ridding the
body of self-cells that are infected by organisms that live within host cells and
self-cells that mutate at the DNA level transforming them into abnormal
and potentially harmful cells. Cell-mediated immunity is critically important in
preventing development of cancer and metastasis after exposure to carcinogens.
HIV and the immune system
Human immunodeficiency virus (HIV) is a retrovirus that gradually destroys the
immune system’s function. When the retrovirus becomes active, the patient
develops acquired immunodeficiency syndrome (AIDS), which is characterized
by profound immunological deficits, opportunistic infection, secondary infec-
tions, and malignant neoplasms.
HIV disables and kills CD4+ T cells, which lowers the immune system’s
capability to fight infection. The number of CD4+ T cells triggers other cells in
the immune system to attack invading organisms. HIV lowers the CD4+ T cell
count and thereby inhibits other immune system cells to go on the attack.
A healthy person who is not HIV positive, has between 800 and 1200 CD4+
T cells per cubic millimeter (mm^3 ) of blood. HIV reduces this count to 200 mm^3.
This is equal to or less than 14%. Infected patients are particularly vulnerable to
opportunistic infections and cancers.
In addition to the T-cell count, the viral load (VL) is a test used to evaluate
the status of the patient’s immune system. The higher the number, the higher the
viral load.
HIV is transmitted in three ways: injection of infected blood or blood prod-
ucts, sexual contact, and maternal–fetal transmission. Occupational exposure to
HIV accounts for a small number of transmissions usually from a needle-stick.
HIV uses three enzymes to genetically encode, replicate, and assemble a new
HI virus within a host cell. HIV can replicate only inside cells. These enzymes
are reverse transcriptase, integrase, and protease.
The HI virus enters the cell through the CD4 molecule on the cell surface.
Once inside the cell, the virus is uncoated with the help of the reverse transcrip-
tase enzyme enabling the virus’ single stranded RNA to be converted into DNA.
CHAPTER 17 Immunologic Agents^317