1. 5 How the immune system works (before HIV infection)

Protection from infection

Your skin is the first main way to protect you from infections.

If your skin is damaged, an infection like HIV can enter your body. For example, through a tiny cut or tear.

HIV can also enter the body through some mucous membranes without needing a cut. Mucous membranes are the moist tissue that lines the vagina or anus and the inner foreskin.

A few infections are transmitted in the air. For example, TB and ‘flu can be breathed in (but not HIV).

Any infections that get past these barriers are then 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 ways to fight an infection.

The two main ways are to use either cellular or humoral (antibody) immune responses. Sometimes these processes and functions overlap.

Cellular immune responses are based on CD4 and CD8 cells.

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.

Humoral immune responses are based on antibodies.

In response to an infection, your body makes antibodies to identify and sometimes destroy the foreign particles.
Once you produce an antibody immune response, your body usually keeps copeis of these antibodies for life.
HIV antibody tests are the commonest way to test for HIV. 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.

Macrophages are another type of white blood cell. Macrophages are larger cells that smother and ‘eat’ 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 hundreds of different types of these surface markers.

You will never need to know about these, but this wiki page lists and briefly describes them all.
https://en.wikipedia.org/wiki/List_of_human_clusters_of_differentiation

Last updated: 1 January 2023.