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PH-IM03

HIV infection

How HIV enters CD4 helper T cells, copies its RNA into DNA and hides in their chromosomes, why CD4 cells decline over years, and why their loss lets opportunistic infections and cancers take hold.

By the end, you can

  • Describe the role of CD4 helper T cells in coordinating adaptive immunity.
  • Name the main parts of the HIV virion and the function of each.
  • Explain how HIV binds CD4 and a co-receptor, and outline its life cycle.
  • Explain why CD4 cells decline during chronic infection and why the infection cannot be cleared.
  • Relate falling CD4 counts to the opportunistic infections and cancers that appear.
  • Recognise the stages, classic tissue appearances, tests and key numbers of HIV infection, and explain how each antiretroviral class blocks a step of the life cycle.

What it is

HIV infection is a chronic infection by a retrovirus that targets cells carrying CD4, chiefly helper T cells. Untreated, it progresses from an acute phase through years of chronic infection to AIDS (advanced HIV disease), in which loss of CD4 cells allows opportunistic infections and cancers.

2 types to tell apart
HIV-1 and HIV-2
HIV-1 causes most infections worldwide. HIV-2, found mainly in West Africa, has a lower viral load, is transmitted less easily and usually progresses more slowly.
CCR5- and CXCR4-using virus
Most transmitted virus uses the co-receptor CCR5; virus that uses CXCR4 often emerges later and is associated with faster CD4 decline.

Normal function

Helper T cells coordinate immunity

Helper T cells carry the surface protein CD4. When an antigen-presenting cell, such as a dendritic cell, shows them a fragment of a microbe, they multiply and release cytokines that coordinate almost every other part of the adaptive response.

They help B cells switch and refine their antibodies, license cytotoxic (CD8) T cells to kill infected cells, and activate macrophages to destroy microbes they have swallowed, such as mycobacteria. Memory CD4 cells, many of them in lymph nodes and the lining of the gut, remember earlier infections for years.

In adults the blood CD4 count is normally about 0.5–1.5 × 10⁹/L (500–1,500 cells/µL), but only a small fraction of the body’s CD4 cells is in the blood at any time; most are in lymphoid tissue.