Human Physiology (UPCAT-Dental)

T-cells are derived from bone marrow stem cells and differentiate into immunocompetent cells in the thymus.

The high-walled endothelium of the post-capillary venules (HEV) in lymph nodes express vascular addressins, which are recognised by homing receptors on lymphocytes passing through the afferent lymphatics and which mediate entry of the lymphocytes into the lymph nodes.

Many PRR are lectin-like in that they recognise exposed microbial sugars.

Endothelial cells are not essentially phagocytic, although they do contribute by the production of mediators of the inflammatory process, particularly acute inflammation and were previously lumped together with the mononuclear phagocyte system in what is known as the reticuloendothelial system (RES).

The defensins are peptide antibiotics present in the granules in extremely high concentration.

T-cells are mainly confined to the paracortical (thymus-dependent) area of lymph nodes, and in nodes taken from children with selective T-cell deficiency. The paracortical region is virtually devoid of lymphocytes.

The granules contain a wide spectrum of microbicidal agents.

These terminal components form a complex, which inserts into the membrane to form a transmembrane ion channel, which leads to lysis of the cell.

Neutrophils are chemotactically attracted to the site of inflammation by C5a and mast cell chemotactic factors such as leukotriene B4.

This is the C3 convertase enzyme generated in the alternative complement pathway and is responsible for splitting off the small peptide C3a, leaving C3b as a residue.

This is a cell matrix component.

Perforin inserts into the target cell membrane, thereby facilitating entry of TNF and granzymes into the target cell.

Molecules such as Bcl-2 and Bcl-XL prevent the release of pro-apoptotic proteins from mitochondria. Note, however, that some members of the Bcl-2 family, such as bid, have pro-apoptotic functions.

The respiratory, alimentary and genitourinary tracts are guarded immunologically by subepithelial accumulations of lymphoid tissues, which are not constrained by a connective tissue capsule. These may occur as diffuse collections of lymphocytes, plasma cells and phagocytes throughout the body and the lamina propria of the intestinal wall or as more clearly organised tissue with well-formed follicles as found in the Peyer's patches.

IgM is the first immunoglobulin class to be expressed on the surface of the developing B-cells, shortly followed by IgD. Early mature B-cells co-express IgM and IgD antibodies of identical antigen specificity.

D regions are only found in the heavy chain; the light chain gene loci lack D segments.

Langerhans' cells in the skin pick up and process antigen, then travel as veiled cells in the lymph before becoming interdigitating dendritic cells in the paracortical T-cell zone of the draining lymph node, where they are potent stimulators of T-cell responses.

Macrophages have an impressive range of functions, but appear unable to prime naive lymphocytes. This can be carried out by lymphoid interdigitating dendritic cells and cultured, but not freshly isolated, Langerhans' cells.

Lymphocytes are directly driven by antigen. The concentration of antigen is important because the binding to the surface of responding B-cells is necessary for continued stimulation. As the antigen concentration falls through catabolism or neutralisation or complex formation with antibody, the immune response diminishes. In this way, the immune response is not maintained at a high level when the antigen has been eliminated.