IgM is the class which appears early following the first encounter with a specific antigen.

The mixed lymphocyte reaction is used for typing MHC class II polymorphic variants.

CD8 + T-cell receptor on the cytotoxic T-cell, recognize the MHC class I + peptide on the target cell surface.

Dendritic cells are potent antigen presenting cells that possess the ability to stimulate naïve T-cells. There are several types of dendritic cells, well distributed over the body.

Centroblasts are found in the dark zone and are highly mitotic cells with no surface IgD and very little surface IgM.

Antigen-reactive cells are depleted from the circulating pool of lymphocytes within 24 h of antigen first localizing in the lymph nodes or spleen, and there is a dramatic fall in the output of cells in the efferent lymphatics.

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

There is very extensive apoptotic cell death amongst light zone centrocytes and the resultant DNA fragments are visible as 'tingible bodies' within the macrophages, which have phagocytosed the apoptotic lymphocytes.

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.

Stimulation of antibody-formation by this thymus-independent antigen leads to proliferation in this location with development of germinal centers while the paracortical region remains inactive, reflecting the inability to develop cellular hypersensitivity to the polysaccharide.

CD8 alpha-alpha homodimers are almost exclusively found on intraepithelial lymphocytes and may restrict these cells to recognition of certain nonclassical MHC molecules, perhaps including TL and Qa1.

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 organized tissue with well-formed follicles as found in the Peyer's patches.

In the absence of costimulator, a cell becomes anergic and incapable of subsequent response to the specific antigen.

The CD4 binds to constant regions on the MHC class II.

B-cells express surface immunoglobulin of a specificity created by that cell's particular immunoglobulin gene recombination. A totally different gene set encodes the T-cell receptor for antigen.