HIV transmission and AIDS

Organisms (Interaction and interdependence) · Defence against disease · note 6 of 8

Spec C3.2.11, C3.2.12Download all Defence against disease notes (PDF)Download all notes (PDF)11 pages

HIV transmission and AIDSSpec C3.2.11, C3.2.12

In short

HIV (human immunodeficiency virus) is transmitted in body fluids such as blood, semen, vaginal fluids and breast milk. It infects and kills only certain types of lymphocyte, the helper T-cells. Losing these limits the ability to activate B-cells and produce antibodies, so the person cannot fight opportunistic infections: this is AIDS.

Transmission of HIV in body fluids

HIV does not survive long outside the body, so it is transmitted in body fluids that pass directly from one person into another's blood or mucous membranes. Mechanisms of transmission include:

unprotected sexual intercourse (semen and vaginal fluids)
sharing hypodermic needles between intravenous drug users (blood)
transfusion of blood or blood products that have not been screened for HIV
from mother to baby across the placenta, during childbirth or in breast milk

Infection of lymphocytes and AIDS

HIV infects only certain types of lymphocyte, mainly helper T-cells, which it enters and eventually kills. Over several years, the number of helper T-cells falls. Because B-cells need contact with activated helper T-cells to be activated, the reduction in these lymphocytes limits the ability to produce antibodies.

The person can no longer fight opportunistic infections: infections by pathogens that a healthy immune system would normally control, such as tuberculosis and pneumonia. This condition is AIDS (acquired immunodeficiency syndrome).

Common mistake:

HIV does not infect all lymphocytes or directly destroy B-cells. Say it kills helper T-cells, so fewer B-cells are activated and fewer antibodies are made.

Written and checked against the IB Biology HL specification · Updated October 2026

Frequently asked questions

How do vaccines work?

Vaccines contain antigens, or DNA or RNA coding for antigens, from a specific pathogen. They trigger a primary immune response in which B-cells are activated and memory cells are produced, without causing the disease. If the real pathogen infects later, memory cells respond quickly, producing antibodies before the person becomes ill.

Why can't antibiotics kill viruses?

Antibiotics work by blocking processes found in bacteria but not in eukaryotic cells, such as cell wall synthesis or protein synthesis by 70S ribosomes. Viruses have no metabolism of their own, so there is nothing for antibiotics to block. They replicate using the host cell's processes, which antibiotics do not target.

What is the difference between innate and adaptive immunity?

The innate immune system responds to broad categories of pathogen and does not change during an organism's life; phagocytes are part of it. The adaptive immune system, based on lymphocytes, responds specifically to particular pathogens and builds up a memory of pathogens encountered, so later responses are faster and more effective.

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