Innate lymphoid cells (ILCs) are populations of lymphocytes that lack TCRs but produce cytokines similar to those that are made by T cells. They are classified into three groups, which produce IFN-γ, IL-5, or IL-17, cytokines that are characteristic of TH1, TH2, and TH17 subsets of T cells, respectively (described later). NK cells are related to group 1 ILCs based on their production of IFN-γ, a cytokine also made by TH1 cells. Because ILCs mostly reside in tissues, they are thought to provide early defense against infections in the tissues, before T cells are activated and can migrate into tissues. ILCs also may be early participants in inflammatory diseases.
The immune system contains several cell types that are specialized to capture antigens and display these to lymphocytes. Foremost among these APCs are dendritic cells, the major cells for displaying protein antigens to naïve T cells. Several other cell types present antigens to lymphocytes at various stages of immune responses.
Dendritic cells (DCs) are the most important antigen-presenting cells for initiating T-cell responses against protein antigens. These cells have numerous fine cytoplasmic processes that resemble dendrites, from which they derive their name. Several features of DCs account for their key role in antigen capture and presentation.
These cells are located at the right place to capture antigens—under epithelia, the common site of entry of microbes and foreign antigens, and in the interstitia of all tissues, where antigens may be produced. DCs within the epidermis are called Langerhans cells.
DCs express many receptors for capturing and responding to microbes (and other antigens), including TLRs and C-type lectin receptors.
In response to microbes, DCs are recruited to the T-cell zones of lymphoid organs, where they are ideally positioned to present antigens to T cells.
DCs express high levels of MHC and other molecules needed for antigen presentation and activation of T cells.
One subset of DCs is called plasmacytoid DCs because of their resemblance to plasma cells. These cells are present in the blood and lymphoid organs, and are major sources of the anti-viral cytokine type I interferon, produced in response to many viruses.
A second type of cell with dendritic morphology is present in the germinal centers of lymphoid follicles in the spleen and lymph nodes and is called the follicular dendritic cell (FDC). These cells bear Fc receptors for IgG and receptors for C3b and can trap antigen bound to antibodies or http://ebooksmedicine.net
Table 5.1 Distribution of Lymphocytes in Tissues *Approximate numbers of lymphocytes in different tissues in a healthy adult.
complement proteins. These cells display antigens to B lymphocytes in lymphoid follicles and promote antibody responses, but are not involved in capturing antigens for display to T cells.
Macrophages (Chapter 3) ingest microbes and other particulate antigens and display peptides for recognition by T lymphocytes. These T cells in turn activate the macrophages to kill the microbes, the central reaction of cell-mediated immunity. B cells present peptides to helper T cells and receive signals that stimulate antibody responses to protein antigens, critical steps in humoral immune responses.