CD4 and neoplasm: Resting B cells express surface immunoglobulin (Ig) as a receptor;8 antigen engagement of surface Ig can activate B cells and leads to internalisation of antigen and presentation of peptides complexed with major histocompatibility complex class II at the cell surface for interaction with CD4+ T cells.8, 9 Lymphocytes are also immune effector cells, and may mediate tumour cell death through granule exocytosis or death receptor signalling, cytotoxic T cells and NKs are key effectors in these mechanisms.