The immune cells are collectively called leukocytes or white blood cells. Immune cells are generally divided into two main types depending on their function: specific cells and non-specific cells.

The specific cells, or lymphocytes, interact with specific micro-organisms that they are programmed to recognise. The non-specific cells depend on molecules produced by the specific cells (such as antibodies) for their activity. Non-specific cells include three sub-groups: phagocytes, dendritic cells and granulocytes. All immune cells are continuously produced in the bone marrow from the same precursor cells, called stem cells. These stem cells mature into many types which are classified by their appearance and function.

Phagocytic cells

Phagocytic cells are able to ingest and destroy foreign material. The main population of cells in this family are the monocytes, found in the blood, and macrophages, found in the tissue. In addition, the neutrophils, basophils and eosinophils belong to a group of cells called granulocytes, which contain granules of toxic substances used to kill bacteria, fungi and large parasites.

Neutrophils circulate in the bloodstream and can migrate into the tissues if necessary. These cells do not live for long and can be produced in enormous quantities in the bone marrow. They are attracted to where they are needed in the body. Sometimes the production of neutrophils is inhibited by drugs. A low neutrophil count is called neutropenia.

Eosinophils are similar to neutrophils and are important for killing larger micro-organisms such as parasites.

Basophils (in the blood) and mast cells (in tissues) are another population of cells in the granulocyte group. However, basophils cannot ingest and destroy foreign material. Basophils are important in responding to inhaled material and gut parasites. They attract other immune cells to sites of foreign material and are involved in allergies, such as hay fever.

Monocytes (in the blood) or macrophages (in tissues) are also a type of 'antigen presenting cell'. Antigen is a foreign substance, usually a protein, that stimulates an immune response. Macrophages present antigen in a particular way to the lymphocytes, thereby helping to induce an immune response. While they can act as antigen-presenting cells, their main role is to envelop and destroy foreign material. Monocytes and macrophages filter foreign material and are concentrated in the liver, spleen, kidney and lymph nodes where blood and lymph fluid pass. These cells live for a long time and carry the CD4 molecule on their surface, and thus can be infected by HIV.

Dendritic cells

Dendritic cells are the main antigen-presenting cells and are more effective at presenting antigen than macrophages. Therefore, dendritic cells are crucial in stimulating an immune response. They are concentrated in the lymph nodes and spleen where they are able to trap foreign material. Dendritic cells also trap material elsewhere, such as the skin and mucosal linings of the bowel and lung. From there they travel to the lymph nodes where they present it to lymphocytes.

Lymphocytes

The third group of immune cells is the lymphocytes. There are three main sub-groups of lymphocytes: the B-, T- and natural killer (NK) cells. B- and T-cells are the most important immune cells for specific or acquired immunity.

B-lymphocytes

B-cells or B-lymphocytes mature in the bone marrow where they are primed to react with only one tiny part of a particular foreign material, called antigen. When presented with that particular antigen, B-cells become activated to release specific molecules called antibodies or immunoglobulins. These are micro-organism-fighting protein molecules. Each antibody attaches itself to a single specific chemical sequence on an antigen.

When a B-cell is activated after recognising antigen, it divides many times making two types of clones. One type is the plasma cell which makes and releases large amounts of the same antibody into the bloodstream. It then dies within a few weeks. The other type is a memory B-cell which persist in the body for many years. If antigen is recognised again, these cells are activated and divide into plasma cells more quickly than in the original response. In this way, the release of specific antibodies becomes more efficient.

There are several classes of antibody called immunoglobulins G, M, A, D and E. Immunoglobulin G (IgG) is particularly useful against bacteria, bacterial toxins and viruses. IgM is the earliest antibody to appear to a newly recognised antigen. IgA is present on surfaces and secretions, such as saliva. IgD is present on the surface of B-cells, expressed as the antigen receptor. IgE is useful against parasites and plays an important role in allergic reactions.

Circulating specific antibody which attaches to specific antigens helps other immune factors and cells destroy the micro-organism. One of these factors is a group of molecules called complement which is produced by the liver. Complement bores holes in the surface of micro-organisms, killing them. When an antibody attaches to an antigen on a micro-organism complement is attracted to the pairing and in this way the antibody brings about the destruction of the micro-organism though attack by complement. This is called humoral immunity.

T-lymphocytes

T-cells or T-lymphocytes mature in the thymus gland situated under the breastbone and possibly in some other lymphoid tissue. They are specifically primed, like B-cells, to react with only one specific antigen and also have receptors on their surface to recognise specific antigen. When a T-cell recognises an antigen, the T-cell is activated and produces clones of itself.

However, instead of releasing antibodies into the bloodstream, the T-cell releases cytokines chemical messengers that attract immune cells towards specific micro-organisms so that the immune cell can destroy the infective agent. The response of T-cells is called therefore called cell-mediated immunity.

CD4 and CD8 T-lymphocytes

T-cells have either CD4 or CD8 receptors on their surfaces. These are molecules which help the T-cells attach to the antigen.

CD4 T-cells are also known as helper cells (Th-cells). Experiments have shown that in other animals CD4 T-cells can be separated into type-1 (Th1) and type-2 (Th2) cells with different functions. In humans there is also a subset of Th0 cells which are thought to be a precursor subset, not yet differentiated into Th1 or Th2.

Th1 cells assist CD8 T-cells and other immune responses against viruses and bacteria inside human cells, enhancing cell-mediated immunity.

Th2 cells assist B-cells in producing specific antibodies by making cytokines which promote activation and growth of B-cells. Therefore, they enhance humoral immunity.

CD8 T-cells are mainly responsible for killing cells that are infected with viruses. Hence, they are important in HIV infection. They are also known as cytotoxic (cell-killing) lymphocytes (CTLs).

Memory, naive and activated T-cells

CD4 and CD8 T-cells include a range of subsets identified by the expression of various cell surface molecules called 'markers'. All T-cells start their lives as 'naive cells' and activate when they meet a particular antigen that they are programmed to recognise. The cells fight the infection and then most die. However a proportion become long-lived memory cells, awaiting the possible reappearance of the same antigen.

These memory T cells mostly express the marker CD45RO and make up most of the initial rise in CD4 and CD8 T-cells after the commencement of antiretroviral therapy. T-cells which express CD45RA are largely 'naive' cells and make up the second, slower phase of the CD4 cell count increase during antiretroviral treatment. However some memory cells may revert to express CD45RA, losing CD45RO. These cells are still in the memory category. Much contention still remains about the precise manner of maturation from naive to memory and the role of such markers as CD45RO and CD45RA.

CD38 and HLA-DR are markers which are expressed when a T-cell is activated. Markers such as these are measured because it is important to have some idea of the proportions of T-cells which are activated, as many of these cells will subsequently die. When T-cells activate they may also express the marker CD95, which is also known as Fas. This molecule increases the T-cells susceptibility to death through the process known as apoptosis or cell suicide.

In HIV infection T-cell activation is elevated too much, and this is associated with high levels of apoptosis or T-cell death. This may be a major contributing factor to the loss of CD4 T-cells and eventually CD8 T-cells during HIV infection.

Natural killer cells

Apart from B- and T-cells, the other type of lymphocyte is the natural killer (NK) cell which is also important in killing virus-infected cells and tumour cells.