Anyone who has applied for a mortgage is familiar with primary lenders, local institutions such as savings and loan associations and commercial banks that make loans and collect monthly payments. Less well known are secondary lenders, multi-billion-dollar organizations that play a key role in the mortgage-financing system.

Suppose a local lender has $5 million available for mortgages. Fifty home buyers, each in need of a $100,000 loan, apply for financing, and every loan application is approved. This is great news for the first 50 people, but what about future borrowers? Has the primary lender run out of money?

To serve additional home buyers, the primary lender needs more money. New funds can be raised by selling local mortgages, such as the 50 loans, into the secondary market. Major players in this market include the Federal National Mortgage Association (Fannie Mae), the Government National Mortgage Association (Ginnie Mae) or the Federal Home Loan Mortgage Corp. (Freddie Mac). Each of these organizations has a distinct method of operation.

Fannie Mae was a governmental agency before being spun off to the private sector. The publicly held organization maintains a loan portfolio that at the end of 1982 included two million mortgages worth a total of $73 billion. In addition, Fannie Mae has created $17 billion in mortgage-backed securities, which may be purchased and resold by investors. Fannie Mae buys conventional, FHA and VA mortgages as well as second trusts and adjustable-rate mortgages.

Ginnie Mae, which is part of the Department of Housing and Urban Development, assembles and guarantees pools of FHA and VA mortgages. Investors may participate in such pools by purchasing "pass-through" certificates on which they receive monthly payments for both interest and principal. At the end of 1982, pass-through certificates worth $119 billion were outstanding.

While the sale and ownership of residential property is usually seen as a local matter, real estate financing is clearly within the stream of interstate commerce. The mortgage on a small house in Houston may well be owned by a pension fund in Boston--a situation which is plausible because secondary lenders have effectively created a national market for mortgage-backed securities and, through those securities, a market for mortgages themselves.

To have a national mortgage market, investors must be able to buy, sell and trade standardized loan products, the value of which can be measured against alternative investments. The secondary lenders have created such products by developing guidelines that define which mortgages they will accept from local lenders. Thus, to have an acceptable, or "conforming," loan that can be resold, primary lenders will tailor their lending practices to meet the standards established by secondary lenders. For instance, one guideline may suggest that no more than 28 percent of a borrower's gross monthly income can be devoted to mortgage payments. Such a guideline then might have a series of exceptions that would allow local lenders some flexibility.

While there are major differences among the three secondary lenders--for example, one is private (Fannie Mae), one deals only with FHA and VA loans (Ginnie Mae) and only one has a major loan portfolio (Fannie Mae)--the national mortgage market created by these secondary lenders has profoundly influenced the entire process of real estate financing. Here's why.

First, the existence of a national mortgage market means that money can move readily from capital surplus areas to regions and lenders that require additional funding. A national mortgage market prevents the Balkanization of the housing industry, a situation where mortgages could be available in one region or state but not others.

Second, the guidelines established by secondary lenders for conventional loans have proven to be in the public interest. Financial qualification standards, for example, protect borrowers, sellers, lenders and mortgage investors, because they assure that loans will be made only to financially able purchasers. In contrast, so-called creative-financing arrangements--where guidelines generally are not employed--have resulted in significant numbers of foreclosures and, it can be surmised, substantial numbers of "house poor" homeowners, individuals who can afford to pay for little other than their mortgage.

Third, a national marketplace creates an element of liquidity, the ability to convert mortgages quickly to cash at a reasonable value. Without a national marketplace, a common ground to buy and sell standardized products, an equal level of liquidity would not exist.

Fourth, a national mortgage marketplace allows local lenders to view their mortgage portfolio as a potential profit center, because loans may be regarded as a commodity to be bought, sold or held advantageously. Moreover, primary lenders also can reap profits by servicing the loans of others--collecting monthly payments--for a fee, usually three-eighths of a percent of the principal balance. In some cases, local lenders may offer loans at especially attractive rates because they want to build up their service business.