The process by which antigen is presented to lymphocytes in a form they can recognise. Most CD4+ T cells must be presented with antigen on MHC class II molecules, while CD8+ Tc cells only recognise antigen on MHC class I molecules. Antigen must be processed into peptide fragments before it can associate with MHC molecules.
Antigen -presenting cells
Antigen-presenting cells (APCs) are a group of functionally defined cells that are capable of taking up antigens and presenting them to lymphocytes in a form they can recognise. Dendritic cells, macrophages, B cells and sometimes even tissue cells can present antigen to Th cells.
A population of polymorphonuclear leukocytes that stain with basic dyes and that have important roles in the control of inflammation.
B cell receptors (BCR)
The B cell surface immunoglobulin that, with its associated signalling molecules, CD79a and CD79b, forms the receptor complex.
Bronchus-associated lymphoid tissue (BALT)
Lymphoid tissue associated with the respiratory tract, i.e. a subset of the mucosa-associated lymphoid tissues.
Cytotoxic T cells
Cells that can kill virally infected targets expressing antigenic peptides presented by MHC class I molecules.
A set of cells present in tissues, which capture antigens and migrate to the lymph nodes and spleen, where they are particularly active in presenting the processed antigen to T cells. Dendritic cells can be derived from either the lymphoid or mononuclear phagocyte lineages.
The parts of an antigen that contact the antigen-binding site of an antibody or the T cell receptor.
Follicular dendritic cells (FDCs)
Antigen-presenting cells present in the B cell areas of lymphoid tissues, which retain stores of antigen.
Helper T cells
A functional subclass of T cells, which can help to generate cytotoxic T cells, cooperate with B cells in the production of antibody responses or can activate macrophages. Helper cells express CD4 and recognise antigen presented on MHC class II molecules. They can be subdivided into Th1 cells, which activate macrophages, and Th2 cells, which promote B cell division and differentiation.
Large granular lymphocytes (LGLs)
A group of morphologically defined lymphocytes containing the majority of NK cell activity. They have both lymphocyte and monocyte or macrophage markers.
Leukocytes that specifically recognise and may react against antigens. The three principal populations are T cells, B cells and large granular lymphocytes (LGLs).
Major histocompatibility complex
A gene locus, referred to as HLA in humans, which encodes a large number of genes, including MHC class-I and class-II genes. These highly polymorphic loci encode cell surface molecules, which present antigenic peptides to T cells.
Cells found distributed near blood vessels in most tissues, which are full of granules containing inflammatory mediators.
A lineage of bone marrow derived long-lived phagocytic cells.
Mucosa-associated lymphoid tissues (MALT)
Generic term for lymphoid tissues associated with the gastrointestinal tract, bronchial tree and other mucosa.
Natural killer (NK) cells
A group of lymphocytes that have the intrinsic ability to recognise and destroy some virally infected cells and some tumour cells.
Polymorphonuclear granulocytes, which form the major population of blood leukocytes.
Peri-arteriolar lymphatic sheath (PALS)
The accumulations of lymphoid tissue constituting the white pulp of the spleen.
Collections of lymphoid cells in the wall of the gut which form a secondary lymphoid tissue.
Cells that can internalise large particles or microbes.
Regulatory T cells (Tregs)
A functionally-defined subset of T cells that express high levels of the IL-2 receptor (CD25) and that are important in controlling immune and inflammatory reactions mediated by T helper cells.
A synonym for 'septicaemia': a life-threatening infection characterised by the circulation and multiplication of bacteria in the blood.
T cell receptor (TCR)
The T cell antigen receptor consisting of either an αβ dimer (TCR-2) or a γδ dimer (TCR-1) associated with the CD3 molecular complex.