anti-Membrane-Spanning 4-Domains, Subfamily A, Member 1 (MS4A1) (C-Term) antibody: CD20 is a 33 kDa surface antigen of leucocytes. It consists of four transmembrane regions and also of cytoplasmic N- and C-termini. CD20 is mainly expressed by B-cells but also by some normal and neoplastic T-cells. Immunohistochemistry (IHC) is a complex technique in which immunological and histological detection methods are combined. In general, the manipulation and processing of tissues before immunostaining, especially different types of tissue fixation and embedding, as well as the nature of the tissues themselves may cause inconsistent results (Nadji and Morales, 1983). Endogenous pseudoperoxidase and peroxidase activity or endogenous biotin and alkaline phosphatase activity can cause non-specific staining results depending on the detection system used. Tissues that contain Hepatitis B surface antigen (HBsAg) can produce false positives when using HRP detection systems (Omata et al, 1980). Insufficient contrast staining and/or improper mounting of the sample may influence the interpretation of results.