How is the blood and immunity systems of humans affected by the aids virus?

The virus begins its life cycle when it binds to CD4 Receptor on T-Lymphocytes. The virus will fuse with this cell and will release its virulent RNA into the cell.

T-Lymphocytes (T Cells) are white blood cells and play a central role in immunity. Different T Cells include Helper T Cells, Cytotoxic T Cells, Natural Killer T Cells, etc. T Cells identify foreign invaders in the body (ie bacteria, viruses) in order to launch an immune response. This is why people with the virus are immune compromised.

In the blood, the white cell count is extremely low in persons with the virus. As a result, the person is unable to protect themselves from simple colds that a healthy person has no problem dealing with. The virus does cross the placenta barrier so it can be transmitted to an embryo. Blood is the number one inoculator for the disease.

First of all, it is not the aids virus. Aids is merely the syndrome that results from infection by HIV. (Human immuno-deficiency virus)

Regardless of semantics, HIV affects immunity by infecting your CD4+ cells. CD4 cells are considered T-helper cells, and are essentially the "generals" of the immune system. They organize your B cells and help organize your CD8 cells as well. This is why people with HIV later develop AIDS, as their immune system becomes unable to respond to invaders in the form of viruses and they can not identify cancerous growths growing inside the body.