The heavy chain of IgA is named the alpha-chain. In serum about 80% of IgA is monomeric, while in secretions, such as saliva, intestinal and bronchial mucus, nasal secretions, sweat, and breast milk and colostrum, the predominant form is the dimeric secretory IgA (sIgA). Besides 4 light-chains and 4 alpha-chains dimeric sIgA also contains a J-chain and a secretory component. The latter protects sIgA against secretory proteases. The Mr of dimeric sIgA is 390 000. The normal B-cell population is polyclonal and expresses a range of different immunoglobulin molecules. In contrast, a majority of B-cell neoplasias are characterized by the proliferation of monoclonal cells expressing only one type of light chain, whereas more than one heavy chain can be expressed by the same cell. The restricted expression of immunoglobulins by monoclonal B-cell lineage proliferations makes antibodies specific for immunoglobulin light- and heavy chains useful for the identification and isotyping of these neoplasias.