ABSTRACT
FIGURE 35 The first defenses against viral infection are located at external and internal body surfaces. (By Ackermann, derived from a study of body proportions by Leonardo da Vinci, 1485-1490, Venice, Galleria dell'Accademia.)
FIGURE 36 Structures of the skin which are of importance in the pathogenesis of viral lesions. The epidermis consists of a layer several cells thick of living cell, the Stratum malpighii (A), covered by a layer of dead keratinized cells, the Stratum corneum (B). The upper cells of the Stratum malpighii contain increasing amounts of keratin granules; sometimes two narrow layers, the Stratum granulosum (C) and the Stratum lucidum (D), are distinguished. The basal layer of dividing epidermal cells is the Stratum germinativum (E). The dermis contains blood vessels (V), lymphatic vessels (L), and fibroblasts and macrophages (FM). The ground substance contains collagenous, reticular, and elastic fibers. The hair follicle (BHF) and sebaceous glands (SG) are appendages of the epidermis. (From Fenner, F., McAuslan, B.R., Mims, C.A., Sambrook, J., and White, D.O., The Biology of Animal Viruses, 2nd ed., Academic Press, New York, 1974, 347. With permission.)
FIGURE 37 Relationship of maternal blood and fetal blood within the human placenta. Maternal blood flows from maternal vessels (below) into open intervillous spaces. Fetal blood (with nucleated red cells) flows through vascular villi separated from the maternal circulation by the trophoblast cells and syncytia of fetal origin. (From Johnson, R.T., Viral Infections of the Nervous System, Raven Press, New York, 1982, 205. © Lippincott Williams & Wilkins. With permission.)
The lymphoid system constitutes the inner defense. It consists of a central component (bone marrow, thymus, spleen) and a peripheral part (spleen, lymph nodes and vessels, unencapsulated scattered tissues such as the tonsils and Peyer's patches in the intestine). The thymus and its equivalent in birds, the bursa of Fabricius, direct immunogenesis in the young and are responsible for cell-mediated immunity. Lymph nodes remove foreign particles and recirculate lymphocytes. The spleen, among other functions, produces lymphocytes and plasma cells. Lymphocytes are constantly recirculated.
