In this lecture, Professor Diamond continues her discussion of hematology by describing the two major blood cell types, erythrocytes and leukocytes.  She begins by reviewing erythrocytes (red blood corpuscles or RBCs) and their characteristics. She discusses differences between RBC counts in men and women and factors like exercise that impact RBC counts. She then contrasts the properties of erythrocytes with those of leukocytes, noting that leukocytes are variable in size, act as a defense mechanism, and vary in numbers depending on the body’s defense needs. She describes how differentiate between different leukocyte types based on the presence of granules in the cytoplasm and the type of nucleus. Professor Diamond surveys the characteristics of leukocyte types, starting with lymphocytes. She discusses the specific cases of B-cells and T-cells and the role that these play in immune response and how T4 cell levels are used in AIDS diagnostic tests. She notes the different T-cell types, including helper, killer, and suppressor T-cells before transitioning to a discussion of monocytes, phagocytic cells that respond to infection. She then discusses granular leukocytes or polymorphs. She starts with the neutrophil, which are the most common white blood cell and also phagocytic. She then discusses acidophyls and their role in allergic reactions, asthma and hay fever. She briefly touches on basophils, and she concludes by mentioning thrombocytes, their function in blood clotting, and their parent cells, megakaryocytes.