phytohaemagglutinin

Phytohaemagglutinin, commonly abbreviated PHA, is a plant lectin found mainly in the seeds of Phaseolus vulgaris (red kidney beans) and in related beans. As a lectin, it binds to specific carbohydrate structures on cell surfaces and can cause agglutination of red blood cells in vitro, hence its name.

In immunology, PHA is a potent mitogen for human lymphocytes in culture, particularly T cells. Under suitable conditions, it stimulates lymphocyte proliferation and is used experimentally to assess lymphocyte responsiveness and to study T-cell function and signaling.

Biological caution and preparation: Phytohaemagglutinin is toxic when ingested in raw or undercooked beans, causing nausea, vomiting, and diarrhea due to its lectin activity. Proper cooking deactivates the toxin; beans should be soaked and thoroughly boiled for an adequate time before consumption. Canned beans are typically safe because processing reduces lectin activity.

Variants and related lectins: Other lectins from Phaseolus beans share similar properties, and several PHA-related proteins can vary in carbohydrate-binding specificity and cell-target preferences. Phytohaemagglutinin is a well-studied example of a plant lectin with both agglutinating and mitogenic activities, illustrating how carbohydrate-recognition proteins influence cell–cell interactions.