Phagocytosis. Colored Scanning Electron Micrograph (SEM) of a cultured macrophage phagocytosing (engulfing) a yeast cell. At right, the white blood cell (blue) is using pseudopodia projections of its cytoplasm to extend towards a yeast spore (yellow). The yeast cell will be swallowed up and digested. Lymphocytes circulate in the blood and play an essential role in the immune response, collecting at sites of infection to repel foreign invaders. Although lymphocytes move by amoeboid locomotion, most are non-phagocytic and instead attack invaders with enzymes. The yeast cell here is similar to the fungus that causes candidiasis (thrush) in humans.