HIV attack and destroy T4 - lymphocytes also known as CD4 cells, are white blood cells that fight infection and play an important role in your immune system. If too many CD4 cells are lost, your immune system will have trouble fighting off infections. Even a minor infection such as cold can be much more severe because the body has difficulty responding to new infections.

**Explanation:**

HIV (Human Immunodeficiency Virus) primarily attacks the immune system cells, specifically the T4 lymphocytes or CD4+ T cells. These cells play a crucial role in coordinating the immune response against pathogens. Let's understand why T4 lymphocytes are the correct answer:

**1. HIV Transmission:**
HIV is primarily transmitted through specific bodily fluids, such as blood, semen, vaginal fluids, and breast milk. The most common modes of HIV transmission include unprotected sexual intercourse, sharing contaminated needles, and mother-to-child transmission during childbirth or breastfeeding.

**2. HIV Target Cells:**
After entering the human body, HIV primarily targets and infects specific immune system cells. The virus binds to CD4 receptors present on the surface of T4 lymphocytes, dendritic cells, and macrophages. However, T4 lymphocytes are the primary target due to the presence of high CD4 receptor density.

**3. Role of T4 Lymphocytes:**
T4 lymphocytes are a type of white blood cell that plays a crucial role in orchestrating the immune response against infections. These cells help in coordinating the activities of other immune cells, such as B lymphocytes and cytotoxic T cells.

**4. HIV Life Cycle:**
Once HIV enters the T4 lymphocyte, it undergoes a series of steps to replicate and spread throughout the body. The virus releases its genetic material (RNA) into the host cell and uses the enzyme reverse transcriptase to convert its RNA into DNA. This viral DNA then integrates into the host cell's DNA, allowing the virus to take control of the cell's machinery for replication.

**5. Destruction of T4 Lymphocytes:**
As the virus replicates within the T4 lymphocytes, it damages and destroys these immune cells. The infected T4 lymphocytes eventually undergo apoptosis (programmed cell death) or become non-functional, leading to a decline in the number and function of T4 lymphocytes in the body.

**6. Immune System Impairment:**
The progressive destruction of T4 lymphocytes by HIV weakens the immune system, impairing its ability to mount an effective immune response against other pathogens. This leads to the development of acquired immunodeficiency syndrome (AIDS), where the body becomes highly susceptible to opportunistic infections and certain types of cancers.

Therefore, option D, T4 lymphocytes, is the correct answer as HIV primarily attacks and destroys these immune cells, leading to immune system impairment and the progression of AIDS.