4. Communication between cellular components of the immune system

Many different types of molecules facilitate communication between cells.

Adhesion molecules

Adhesion molecules make it possible for two cells to communicate directly. Like a form of “handshake”, two cells each contain molecules that bind to each other and induce changes inside the cell. This interaction does not always depend on antigens. There are five families of adhesion molecules.

Ig-superfamily

The first family is the Ig-superfamily. It contains molecules like CD4 and CD8, which we already know the function of, but other molecules as well. B7-1 and B7-2 are both members of the family and are found on antigen-presenting cells. CD28 and CTLA-4 are also members and are found on T-cells. The latter two can bind to the former two and this binding is important in T-cell activation by antigen-presenting cells. While CD28 stimulates T-cell activation, CTLA-4 inhibits it, to dampen its activity.

Selectins

The second family are the selectins. This family is divided in three. There exists L-selectin on lymphocytes, E-selectin on endothelial cells and P-selectin on platelets. They are important for extravasation which we will see in a later chapter, and homing, the process of transporting leukocytes to lymphoid organs and inflammation.

Integrins

The third family are the integrins. Their main function is attachment of cells to extracellular matrix. The ligands of integrins can be proteins found in ECM, like collagen. During inflammation, integrins on phagocytes can bind to integrin ligands found on the endothelium.

Muclin-like molecules

The fourth family are the mucin-like molecules, but they’re not important.

"Others"

The last family are the “other” accessory molecules. One member of this family is CD45, a protein found on all leukocytes. It regulates T-cell activation. One subtype of it, CD45RA, is found on naïve T-cells, while another, CD45R0, is found on memory T-cells. Other important “other” molecules are CD25, which is the receptor for IL-2, and CD154, which is the ligand for CD40.

Neighbouring cells can also communicate by connecting with microtubules or by sending microvesicles to each other, but not much is known about this communication.

Soluble molecules. Cytokines.

You have read about several cytokines already. All interleukins and interferons are cytokines. The majority of interleukins are produces by CD4+ T-cells. Cytokines can act on the cells that produce them (autocrine), on nearby cells (paracrine) or on cells reached through the circulation (endocrine). They can have different functions on different cell (pleiotropy), many cytokines can have the same effect on the same cell (redundancy), the effects of to cytokines can be stronger than the sum of their effects (synergy) and cytokines can inhibit the effects of other cytokines (antagonism).

Cytokines are used to send information between cells, and to regulate the immune response. They act through specific receptors that have high affinity. These receptors trigger signal transduction. Cytokines are often found in very low concentration.

Chemokines are small (~100 amino acid polypeptides) that enhance cell adhesion, and recruit cells to inflammatory tissue.