Adaptive immunity. There are two types of adaptive immunity: humoral and cell-mediated. Humoral immunity is mediated by antibodies that are produced by B lymphocytes. It is the principal defense mechanism against extracellular microbes and their toxins, with secreted antibodies binding to microbes and toxins to assist in their elimination. Cell mediated immunity is mediated by T cells, with dendritic cells playing important roles in antigen presentation. T cells can function by various methods: (1) activating macrophages to kill phagocytosed microbes; (2) directly destroy infected cells; and (3) by releasing cytokines and alter the milieu around them. (Reproduction of this figure in color was made possible by Texas Biotech Corp., Houston, Texas.)