During an inflammatory response adhesion molecules serve to enhance pairing between many less avid receptors and their ligands and transmit signals that direct specific effector functions. At least four superfamilies of adhesion molecules participate in these events: the selectins, the integrins, certain members of the immunoglobuline superfamily and cadherins.
The
selectin family is composed of three members named
according to the cells in which they were originally discovered.
L-selectin (CD62L) is constutively expressed on leukocytes
and its target cells are activated endothelial cells.
E-selectin (CD62E) is produced exclusively by endothelial cells
after cytokine activation and its counter-receptors are on
neutrophils, monocytes, eosinophils, lymphocyte subsets and some
tumour cells. P-selectin (CD62P) is preformed and stored for
rapid release in the granules of platelets or the Weibel-Palade
bodies of endothelial cells but in the latter P-selectin is
expressed only after cytokine activation. Its target cells are the
same as those for E-selectin. Each selectin receptor molecule
contains a lectin-like N-terminal domain, followed by an epidermal
growth factor--like motif, a series of consensus repeats similar
to those in complement-regulatory proteins, a transmembrane
domain, and a cytoplasmic tail. The lectin domain is directly
involved in mediating cell-cell contact through 
-dependent
interactions with cell-surface carbohydrates, particularly the
sialyated Lewis X antigen (sLe
).
The
integrins are a large family of heterodimeric
glycoproteins which can be subdivided according to the particular
subunit they possess, which is shared by all members of the
group. On this basis, this family can be subdivided into the ,
, and integrins. The
integrins are expressed particularly
by leukocytes, giving rise to their alternative name, the
leukocyte integrins, whereas the others, in general, are more
widely distributed. The
leukocyte integrins are
represented by three heterodimeric molecules:
LFA-1
(CD11a/CD18), CR3 (DC11b/CD18), and CR4 (CD11c/CD18). Each
of them contains the same 95kDa integrin subunit, also
designated CD18 and different 
chains designated CD11a (180 kDa),
CD11b (165kDa), and CD11c (150kDa). The intact CD11a/CD18 molecule
is the lymphocyte function-associated antigen-1, LFA-1 and is
expressed by lymphocytes (including T cells), myeloid cells
(monocytes, macrophages, and granulocytes), and a variety of other
cell type. CD11b/CD18 is the complement receptor type 3, CR3, and
CD11c/CD18 is CR4 (also called p150,95). Both CR3 and CR4 are
expressed by myeloid cells.
There are two or more ligands for LFA-1, those defined to date being ICAM-1 (CD54) and ICAM-2 (CD102), which are members of the immunoglobulin superfamily. CR3 can bind fragments of complement components, particularly iC3b, and it mediates phagocytosis of complement-coated particles by professional phagocytes.
Alongside with intercellular adhesion molecules
ICAM-1
and
ICAM-2 there is additional member of the diverse
immunoglobulin superfamily - platelet-endothelial cell adhesion
molecule,
PECAM-1 (CD31).
Both ICAM-1 and ICAM-2 are
expressed by endothelial cells, PECAM-1 has been indentified on
neutrophils, monocytes, platelets and is present in large amounts
on endothelial cells
(about 
copies per cell), where it is
concentrated at cell-cell junction.
The main adhesion molecules participating in the interactions between neutrophils and endothelial cells of postcapillary venules are shown in Table 1.13).
Table 1.13: Adhesion molecules involved in leukocyte
binding to endothelium