1. 5 How the immune system works (before HIV infection)
Protection from infection
Your skin is a major barrier to infections.
If your skin is damaged, for example through a tiny cut or tear, an infection like HIV can enter your body. HIV can also enter through some mucous membranes without needing a cut. Mucouds membranes are the moist tissue that lines the vagina or anus and the inner foreskin of the penis).
A few infections can also be transmitted in the air by being breathed in (like influenza or TB).
If an infection enters the body it is attacked by the immune system.
Antigens and antibodies
Two medical words are often used when talking about the immune system and infections:
- Antigen is a word for a small particle of infectious material that has been broken down in the body and recognised by the immune system.
- Antibody is a type of protein made by certain white blood cells in response to a foreign substance (antigen) in the body. Each antibody binds to only one specific antigen. The purpose of this binding is to help destroy the antigen. Some antibodies destroy antigens directly. Others make it easier for white blood cells to destroy the antigen.
Cellular and humoral immunity
Your body uses different cells to attack and break down an infection. There are two main ways that your body does this:
Humoral immunity is based on antibodies.
- In response to an infection you body makes antibodies to identify and sometimes destroy the foreign particles.
- HIV is routinely diagnosed using an antibody test. This looks for evidence of the body’s response to HIV. This response usually takes 2-3 weeks from infection to develop, but can take several months and occasionally longer.
Cellular immune responses are based on CD4 and CD8 responses.
Generally your body uses cellular immunity to fight viruses, and to fight HIV.
- T cells are a type of white blood cell (lymphocyte). The two main types of T cells are CD4+ cells and CD8+ cells (T4 and T8 cells). The plus sign is used in medical literature but in everyday language they are referred to as CD4 and CD8 cells.
- CD4 cells are sometimes called helper cells because they help the immune response by sending signals to CD8 cells.
- CD8 cells are sometimes called killer cells because they recognise and kill cells that are infected with a virus
Sometimes these cell processes and functions overlap.
Macrophages are another type of larger white blood cell. Macrophages engulf or swallow up infectious organisms or waste material from dead cells. They also send signals to activate other cells in the immune system.
What does ‘CD’ stand for?
CD stands for cluster of differentiation. Cells in the immune system are classified by these molecules (glycoproteins), which are found on the surface of the cell.
The number after CD (ie CD4, CD4, CD38 etc) is used to differentiate between the many different types of these surface markers.
Last updated: 1 January 2016.