How HIV causes the symptoms of AIDS. Why antibiotics are ineffective…

The idea

HIV replication destroys helper T cells. Because these cells release cytokines that stimulate phagocytes, cytotoxic T cells, and B cells, their depletion collapses the specific immune response. Consequently, B cells are poorly activated, reducing plasma cell and antibody production. The patient cannot fight off pathogens, leading to AIDS. Symptoms are caused by opportunistic secondary infections (e.g., tuberculosis), not HIV directly. AIDS is diagnosed by a low CD4 count or an AIDS-defining illness, as an antibody test only shows infection. Antibiotics are ineffective against viruses because viruses lack the bacterial structures and metabolic processes (like murein cell walls) that antibiotics target.

What earns the marks

• State that HIV replication destroys helper T cells, leading to a significant fall in their numbers.
• Explain that fewer helper T cells means reduced activation of B cells and cytotoxic T cells, resulting in decreased antibody production.
• Attribute the symptoms of AIDS to secondary or opportunistic infections rather than the direct action of HIV.
• Note that diagnosis of AIDS requires a low helper T cell (CD4) count or an AIDS-defining illness, not simply the presence of antibodies.
• Clarify that antibiotics are ineffective against viruses because viruses lack bacterial structures or metabolic processes, such as a murein cell wall or ribosomes, for the drugs to target.

Common mistakes and how to put them right

• Stating HIV causes the symptoms of AIDS directly, instead of through infections the weakened immune system cannot fight.

  Put it right: Attribute symptoms to opportunistic secondary infections.

• Writing that antibiotics cannot get inside the virus, rather than that viruses lack the bacterial structures and metabolic processes antibiotics disrupt.

  Put it right: Name a bacterial target such as the murein cell wall or ribosomes that viruses lack.

• Confusing antibiotic resistance with immunity.

  Put it right: Use 'resistant' for bacteria and 'immune' for the host.

• Treating a positive HIV antibody test as proof that a person has AIDS.

  Put it right: State that AIDS requires a low CD4 count or an AIDS-defining illness.

Exam tips

• When asked why antibiotics are ineffective against viruses, always name a specific bacterial target the virus lacks, such as a murein cell wall or metabolic enzymes.
• Open an AIDS pathogenesis answer by attributing the symptoms to secondary infections, then explain the underlying fall in helper T cells that prevents antibody production.
• If a question asks what else is needed to diagnose AIDS, give a low helper T cell count or an AIDS-defining illness, not just an antibody test.