The skin is a specialized lymphoid organ, called “SALT” or “skin-associated-lymphoid-tissue”. There are 1 million T cells per square inch of skin and their dysfunction can result in disease.
In this 3-part blog series, we discuss T cells, dendritic cells, and psoriasis disease. Cells that make up our peripheral blood mononuclear cells (PBMC) arise through cell division of resident hematopoietic stem cells in the bone marrow, a primary lymphoid organ. T cells are presented with pathogens by antigen presenting cells in the secondary lymphoid organs, which include the spleen, lymph nodes, and other peripheral tissue.
The skin is the largest organ in the human body and the first line of defense from pathogens. Not surprisingly, the skin has been recognized as a specialized lymphoid organ. It is called “SALT” or “skin-associated-lymphoid-tissue”. Normal skin contains a large population of resident memory T cells (T rm). These T cells are recognizable from other T cells through their expression of the CD3 marker and CLA receptor. It is estimated that there are one million T cells per square centimeter of the human skin.
The skin consists of three layers: epidermis, dermis, and adipose. Keratinocytes (skin cells in the epidermis) release a chemical signal (chemokine CCL27) to attract T cells to healthy skin for immune surveillance. Almost 10% of T cells from PBMCs become specialized for skin protection and these T cells maintain steady-state skin immunity in accord with other cells of the PBMCs. This is known as a state of tolerance.
When resident T cells in the skin sense danger, they produce chemical signals, or cytokines, which are a broad range of proteins important in cell signaling. They in turn activate genes for the production of chemokines, which are chemotactic cytokines or chemical signals that attract other cells. Of course, the responding cells are equipped with receptors for chemokine detection. In this manner T cells are able to recruit more T cells. This starts an immune response.
There is a reason why T cells’ signaling to recruit more T cells does not happen through a direct response. Turning on one signal to turn on another signal to get a response can be likened to having check points to ensure that there is control. Dendritic cells are involved, which we will discuss in Part II of this 3-part blog series. But despite checkpoints in place, sometimes communication does break down and in Part III, we discuss the disease state that ensues. HemaCare is a trusted source for T cells, including CD3+ Pan T cells, CD4+ Helper T cells, and CD8+ Cytotoxic T cells.
. Lowes MA, Suárez-Fariñas M, Krueger JG. Immunology of psoriasis. Annu Rev Immunol. 2014 Mar 21;32:227-55. PubMed PMID: 24655295.