CD4+ T-Lymphocytes

Princess Leia representing a naive CD4+ T-Lymphocyte Princess Leia represents a naive CD4+ helper T-lymphocyte, unsure how to tackle the imperial threat but willing to help direct other troops.

CD4+ T-lymphocytes mature in the thymus and form one of the major types of T-lymphocytes, the other being CD8+ T-lymphocytes. CD4+ T-lymphocytes are also known as helper T-cells, as their name suggests they 'help' other cells in the immune system carry out their job.

Unlike antibodies, the T-Cell Receptor (TCR) cannot bind to an antigen directly, instead it requires a fragment of the antigen to be presented via a complex called Major Histocompatibility Complex (MHC). Two classes of MHC exist, but CD4+ T-lymphocytes recognise and bind to MHC Class II on professional antigen presenting cells, such as macrophages and dendritic cells. Once this happens the CD4+ T-lymphocytes undergo clonal expansion, producing multiple copies of itself. Some clones become long-lived memory cells that help the immune system remember threats that have already been encountered and respond more quickly and robustly the next time the same threat is present.

CD4+ can be further classified as T-helper type 1 (Th1), T-helper type 2 (Th2), Th17 or Regulatory T-lymphocytes. Th1 cells are involved in the defence against intracellular pathogens, such as bacteria and viruses. Th2 cells defend against large extracellular organisms, such as parasites or worms. Th17 cells are involved in the immune response to fungal infections, whilst regulatory T-lymphocytes, or Tregs, help maintain tolerance and homeostasis, or balance, within the immune system.

A large part of the differences between these cell types is the chemical messengers, or cytokines that they secrete. Cytokines act to provide signals to other cells telling them what to do, with different combinations of cytokines resulting in different types of response that are most appropriate for the threat being encountered. An imbalance in the Th1/Th2 cell types can lead to autoimmune conditions if the Th1 response is dysregulated or allergy and asthma if the Th2 response is aberrant



Soluble Mediators