Ans. is 'b' i.e. PNH Cancer cells express a variety of antigen.These antigens can be targeted by certain antibodies which are specifically directed towards those antigens. The antigen antibody reaction leads to death of the cancer cells.Antibodies {monoclonal antibodies) are now being prepared by immunizing mice against human tumour cells and obtaining.The antibodies so obtained have a short-half life and induce human anti-mouse antibody immune response. They are usually chimer zed or humanized when used as therapeutic reagents.Presently several monoclonal antibodies have received FDA approval for t/t of tumours. These includeRituximabAlemtuzumabTrastuzumabMechanism of action of the monoclonal antibodies:-The clinically relevant mechanism of action is not clear but certain other reasons have been postulated which areAntibody dependent cellular toxicityComplement dependent cytotoxicityDirect induction of apoptosisRituximabRituximab is a chimeric antibody that targets CD 20 B cell antigen.CD-20 is present on the cells from Pre-B cell stage and is expressed on 90% of B cell neoplasms.Rituximab is the first monoclonal antibody to receive FDA approval.It is primarily used in the t/t of lymphomas.Binding of Rituximab to CD20 generates transmembrane signals that produce autophosphorylation and activation of serine/tyrosine protein kinases.Uses of RituximabLymphoma (B cell lymphomas)Low grade lymphomasMantle cell lymphomasRelapsed aggressive B cell lymphomasChronic lymphocytic leukemiaSLE (Harrison 17th/e p 2082 Fig. 313.3)(used in patient with severe SLE)Rheumatoid arthritisRituximab has shown benefit in the treatment of rheumatoid arthritis refractory to anti TNF agentsRituximab has been approved for the treatment of active rheumatoid arthritis when combined with methotrexate.