Processed antigen reacts selectively with lymphocytes bearing .appropriate surface immunoglobulin molecules resulting in the clonal proliferation of these cells, produc­ing circulating immunoglobulin (humoral immunity) or non-circulating immunoglobulins (cellular immunity). 
High levels of circulating immunoglobulins (lgM & lgG) occur in protozoa and helminths infections. Reaginic antibodies attach to mast cells and basophils in the pre­sence of specific antigen mediate mast cell/basophil de­􀁛ranulation which results in the release of vaso-active amines. The process is initiated when antigen becomes attached to the combining site of the cells band lgE, inf­luencing the activity of a cell membrane-associated enzyme system. 
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