Protein‐carbohydrate interactions serve multiple functions in the immune system. Many animal lectins (sugar‐binding proteins) mediate both pathogen recognition and cell‐cell interactions using structurally related Ca²⁺‐dependent carbohydrate‐recognition domains (C‐type CRDs). Pathogen recognition by soluble collections such as serum mannose‐binding protein and pulmonary surfactant proteins, and also the macrophage cell‐surface mannose receptor, is effected by binding of terminal monosaccharide residues characteristic of bacterial and fungal cell surfaces. The broad selectivity of the monosaccharide‐binding site and the geometrical arrangement of multiple CRDs in the intact lectins explains the ability of the proteins to mediate discrimination between self and non‐self. In contrast, the much narrower binding specificity of selectin cell adhesion molecules results from an extended binding site within a single CRD. Other proteins, particularly receptors on the surface of natural killer cells, contain C‐type lectin‐like domains (CTLDs) that are evolutionarily divergent from the C‐type lectins and which would be predicted to function through different mechanisms.