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One of the body's defenses when it is under attack is circulating antibodies in the blood that latch on the antigens on the surface of the yeast cell. We are not talking about just one antigen and one antibody attached to it, per yeast cell. There can be literally hundreds of such pairs per yeast cell. Presence of such Ag-Ab pairs gets the complement systems going, and pretty soon the cell is coated with literally thousands of particles of complement. (Please read my previous article on "Complement (no typo) to your health" for background information on complement system and how it works).
Cells which are covered by complement fragments are said to be "opsonized". Such cells are quickly killed. Phagocytic cells (cells that are literally capable of eating up other cells) such as macrophages and neutrophils are attracted to the site of the opsonized cells, and quickly eat them up. This is called the classic pathway by which complement attacks pathogens such as yeast cells.
Tumor cells grow in the body in the first place because the body does not recognize them as "foreign and dangerous", as are yeast cells. Part of the problem is that tumor cells do not have a real ugly antigen on their surface that triggers activation of the immune system. If this were not the case, there would be sufficient home grown antibodies against the cancer cells, and the tumor would be killed before it had a chance to grow. The fact you have CLL says your body is not able to mount a sufficiently effective antibody response to the antigens on the tumor cells.
OK. So we have CLL, but no antigen that the body recognizes, and therefore no antibody. Not to worry, IDEC comes to the rescue, we define CD20 marker on B-cells as an antigen, and make an antibody against it called Rituxan. Once this drug is administered to patients with CLL, especially those with a large percentage of b-cells that are positive for CD20, and have a large number of CD20 markers per cell, we have met the first condition for killing the cell. Namely, antigen-antibody complexes are formed on the surface of the cancer cells.