The effect of prior exposure to mercuric chloride (HgCl2) on serum anti-Chlamydia antibodies and anti-collagen antibodies in Chlamydia-induced arthritis. (a) HgCl2-exposed rats had significantly higher serum IgG1 anti-Chlamydia antibodies than non-exposed control rats (P < 0.001). Control rats had slightly higher IgG2a anti-Chlamydia antibodies than the HgCl2-exposed rats (P = 0.12). (b) HgCl2-exposed Brown Norway (BN) rats had higher levels of IgG antibodies to rat type II collagen than controls (P < 0.01) (n = 8 in each group). O.D., optical density.