Serum Blocking Factors 1, 2, 3

ABSTRACT-Blocking factors in the sera of inbred CBA mice bearing either Moloney murine sarcoma virus-induced tumors or transplanted 3-methylcholanthrene-induced tumors specifically blocked in the leu-kocyte adherence inhibition reaction between tumor-sensitized perito-neal cells and tumor antigens. These factors were absorbed from serum by treatment with anti-Ia antibodies and could be eluted in a partially purified form. Chromatography on Sephacryl S-200 columns demonstrated blocking activity in fractions of high and low molecular weight (>100,000 and 40,000-50,000, respectively) derived from whole sera or eluates. Active fractions from serum appeared to retain the tumor specificity of the original material.-JNCI 1982; 69:939-944. Specific BF have been defined as any humoral factors that can in a specific way impair an immune response to tumor antigens independently of their site of action (central