T follicular helper cells (Tfh) are a specialized subset of CD4+ T cells that were first identified in the human tonsil. They play a critical role in protective immunity helping B cells produce antibody against foreign pathogens. Tfh are located in secondary lymphoid organs (SLOs), including the tonsil, spleen and lymph nodes. These organs contain numerous lymphocytes, separated into defined T and B cell zones.
CD4 T cells play a key role in the functioning of a healthy immune system. They assist B cells to make antibodies, activate the microbe killing capacity of macrophages and recruit other immune cells to infected or inflamed areas of the body. These activities are orchestrated through their production of various cytokines and chemokines.
T cells are so called because they are predominantly produced in the thymus. They recognise foreign particles (antigen) by a surface expressed, highly variable, T cell receptor (TCR). There are two major types of T cells: the helper T cell and the cytotoxic T cell. As the names suggest helper T cells ‘help’ other cells of the immune system, whilst cytotoxic T cells kill virally infected cells and tumours.
As the name suggests regulatory T cells (also called Tregs) are T cells which have a role in regulating or suppressing other cells in the immune system. Tregs control the immune response to self and foreign particles (antigens) and help prevent autoimmune disease. Tregs produced by a normal thymus are termed ‘natural’. Treg formed by differentiation of naïve T cells outside the thymus, i.e. the periphery, or in cell culture are called ‘adaptive’.
Invariant natural killer T (iNKT) cells, also known as type I or classical NKT cells, are a distinct population of T cells that express an invariant aβ T-cell receptor (TCR) and a number of cell surface molecules in common with natural killer (NK) cells.
Natural Killer (NK) Cells are lymphocytes in the same family as T and B cells, coming from a common progenitor. However, as cells of the innate immune system, NK cells are classified as group I Innate Lymphocytes (ILCs) and respond quickly to a wide variety of pathological challenges. NK cells are best known for killing virally infected cells, and detecting and controlling early signs of cancer. As well as protecting against disease, specialized NK cells are also found in the placenta and may play an important role in pregnancy.
Mast cells are long-lived tissue-resident cells with an important role in many inflammatory settings including host defence to parasitic infection and in allergic reactions. Mast cells are located at the boundaries between tissues and the external environment, for example, at mucosal surfaces of the gut and lungs, in the skin and around blood vessels.
Macrophages are specialised cells involved in the detection, phagocytosis and destruction of bacteria and other harmful organisms. In addition, they can also present antigens to T cells and initiate inflammation by releasing molecules (known as cytokines) that activate other cells.
Figure 1. Lung macrophages stained with Wright-Giemsa
Gamma delta (γδ) T cells are the prototype of ‘unconventional’ T cells and represent a relatively small subset of T cells in peripheral blood. They are defined by expression of heterodimeric T-cell receptors (TCRs) composed of γ and δ chains. This sets them apart from the classical and much better known CD4+ helper T cells and CD8+ cytotoxic T cells that express αβ TCRs. The mechanism of (thymic) selection of γδ T cells is still largely unkown.
Eosinophils are major effector cells in the immune system. They have a beneficial role in host defence against nematodes and other parasitic infections and are active participants in many immune responses. However, eosinophils can also be damaging as part of the inflammatory process of allergic disease.
