Is a receptor found on helper T cells and monocytes. The CD4 receptor on helper T cells binds to HLA class II and the interaction causes CD4 helper T cell activation. CD4+ T cells coordinate the immune response by releasing cytokines and signalling to other cells via cell-cell contact that enables others immune cells to perform their specialised functions (such as B cells). The number of CD4 cells in a sample of blood is an indicator of the health of the immune system. A normal CD4 count ranges from 500 to 1500. HIV infects and kills CD4 cells, leading to a weakened immune system and lower CD4 counts.

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