Risks underlying Alt-A residential mortgage-backed securities prompted Fitch Ratings to downgrade thousands of loan classes this week. At the same time, the agency affirmed thousands of Alt-A RMBS from the same series of deals. The ratings actions stem from Fitch's ongoing review of  collateral backing Alt-A RMBS and the risks posed by current market conditions. The downgrades come at a time when extended foreclosure timelines in certain states are disrupting the mortgage marketplace, leading to higher expectations for loss severities. Fitch downgraded 2,445 classes from 764 Alt-A RMBS deals this week. The affected transactions were issued between 1996 and 2008. Analysts also affirmed 7,784 classes of Alt-A RMBS securitizations from the same batch of deals. Prior to the downgrades, most of the affected classes were already listed as non-investment or distressed ratings. Fitch said expected loss severities on the most recent vintage of loan pools jumped 5% to to 10% on average due to pressures stemming from the extended foreclosure timelines and volatile home prices. Write to Kerri Panchuk.