Abstract • 19

Lymphocytes and antigen-presenting cells develop from precursors in bone marrow through an ordered series of steps whereby they express receptors for antigen on their surfaces. The genes responsible for these receptors are not contiguous and in the case of B- and T-cells must be joined or recombined by enzymes called recombinases. The antigen receptors on the two types of lymphocytes, T-cells and B-cells are related. Following receptor expression, antigen binding provides an activation signal to the nucleus that antigen has been encountered and preparation for proliferation (to provide more copies of themselves for antigenic encounter) and the development of specialized functions such as the secretion of small hormones called interleukins should begin. B-cells recognize antigen in solution. T-cells recognize antigen bound to histocompatibility (HLA) antigens of type I & II. The antigen receptors themselves do not signal themselves accessory molecules do this in both B-cells. Signaling occurs via a group of enzymes, which place phosphate groups on tyrosine residues in proteins (protein tyrosine kinases, PTKs). Signals received in the nucleus start the production of interleukins and also initiate production of specific receptors for these interleukins, which are then expressed, on the cell surface. The cytokines produced by activated lymphocytes bind to specific cell surface receptors and signal the nucleus again. Specific cell surface receptors are expressed. Binding of an interleukin to its receptor signals the nucleus through PTKs so that proliferation and differentiation occur. The primary immunodeficiency diseases will be discussed in the context of these processes.