AIDS, short for Acquired Immunodeficiency Syndrome, is a chronic and potentially life-threatening condition caused by the Human Immunodeficiency Virus (HIV). HIV attacks and weakens the immune system, making the individual susceptible to various infections and diseases. It is characterised by a reduction in the number of T4-lymphocytes or CD4 cells, which are a type of white blood cells that play a crucial role in the immune response.

HIV and the Immune System
- HIV primarily targets CD4 cells, which are crucial for coordinating the immune response against infections.
- The virus enters the CD4 cells, replicates itself, and destroys the host cell in the process.
- As the virus continues to attack and destroy CD4 cells, the immune system becomes progressively weaker, leading to AIDS.

Reduction in T4-Lymphocytes
- T4-lymphocytes, also known as helper T cells, are responsible for coordinating the immune response by releasing chemical messengers called cytokines.
- These cytokines help in activating other immune cells, such as B cells and cytotoxic T cells, to fight off infections.
- In HIV infection, the virus directly attacks and destroys CD4 cells, leading to a gradual decrease in their numbers.
- The depletion of CD4 cells weakens the immune response and makes the individual more susceptible to opportunistic infections and certain types of cancer.
- CD4 cell count is an essential marker used to determine the progression of HIV infection and the onset of AIDS.

Other Symptoms and Complications
While the reduction in T4-lymphocytes is a hallmark characteristic of AIDS, there are other symptoms and complications associated with the condition:
- Recurrent infections: HIV weakens the immune system, making individuals prone to frequent and severe infections, such as pneumonia, tuberculosis, and fungal infections.
- Opportunistic infections: These are infections caused by organisms that generally do not cause disease in individuals with a healthy immune system. However, in people with AIDS, these infections can be severe and even life-threatening.
- Cancers: AIDS increases the risk of certain types of cancers, including Kaposi's sarcoma, non-Hodgkin's lymphoma, and cervical cancer.
- Wasting syndrome: AIDS can lead to severe weight loss and muscle wasting, known as wasting syndrome.
- Neurological complications: HIV can also affect the central nervous system, leading to cognitive impairment, memory problems, and neuropathy.

In summary, AIDS is characterised by a reduction in the number of T4-lymphocytes or CD4 cells, which weakens the immune system and makes individuals susceptible to various infections, cancers, and other complications.