Final answer:
When helper T cell counts fall below 200 cells/μL in the blood, it signifies the progression to AIDS, leading to severe immunodeficiency and high susceptibility to opportunistic infections, which could be fatal.
Step-by-step explanation:
When helper T cells fall below about 200 cells per cubic milliliter of blood, this condition is indicative of the progression to AIDS (Acquired Immunodeficiency Syndrome) from HIV (Human Immunodeficiency Virus) infection. At this critical threshold, the immune system is severely compromised, and the individual becomes extremely susceptible to opportunistic infections—infections that are typically rare in people with a healthy immune system. These opportunistic infections can be the immediate causes of death in people with AIDS. It is important to monitor CD4+ T cell counts in HIV-infected patients using flow cytometry so that treatment can be adjusted accordingly. The fall in T cell count below the critical level means that cell-mediated immunity is lost, leading to an increased risk of opportunistic infections and diseases that the body would normally be able to fend off.