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Role of Immunization
Melissa Kennedy, DVM, PhD, DACVM

Department of Biomedical and Diagnostic Sciences, College of Veterinary Medicine, University of Tennessee, Knoxville, TN, USA

The immune system is a complex network of interacting order to design or select the most appropriate antigen
cells and molecules. Manipulation of the immune delivery method.
response is an attractive prospect, and has been achieved B lymphocytes are able to detect free and soluble anti-
in part through immunization. Immunization is the pro- gens, while T lymphocytes are only able to recognize
cess of exposing an animal to whole or parts of microbes antigens presented to them “on the pedestal” of the major
in order to stimulate an acquired response. Immunization histocompatibility complex (MHC) structures. For the
takes many forms, and continues to advance as a field of latter to occur, the foreign antigen must be processed
research. This chapter focuses on several aspects of within a cell and displayed on the MHC for T lymphocyte
immunization in veterinary medicine. recognition. MHC structures are classified as class I or
Before one can understand immunization, one must class II. The former are expressed on all nucleated cells
understand the nature of the acquired or adaptive immune and express only antigens that are produced within that
response. Acquired immunity is that which is developed cell. The cytotoxic T lymphocytes recognize antigens
after an animal is born and is restricted primarily to the T presented in MHC I. MHC class II are expressed on anti-
and B lymphocytes. This is to distinguish it from innate gen‐presenting cells (APCs), such as macrophages, den-
immunity with which an animal is born. This latter form dritic cells, and B lymphocytes. The antigen displayed by
of immunity is nonspecific in nature and involves many these structures is produced outside of and taken up
cell types and molecules such as cytokines and comple- by the APCs via endocytosis or phagocytosis. The anti-
ment. Acquired immunity is distinct in that it has a high gen is then processed and inserted in MHC II for presen-
degree of specificity and diversity of antigen recognition, tation to T helper lymphocytes. Understanding how
as well as memory. It is this latter trait that is exploited by lymphocytes differ in this antigen recognition process is
immunization – the ability to establish an immunologic critical to designing and selecting efficacious vaccines.
memory of an encountered foreign antigen. B lymphocytes bear immunoglobulins on their surface
The major cells involved in the specific immune for antigen recognition. As mentioned above, these anti-
response, and the only ones that manifest these proper- gens are extracellular, such as free‐living bacteria and
ties of adaptive immunity, are the lymphocytes. Different parasites and the extracellular form of viruses and other
lymphocyte lineages are distinguished by cell surface intracellular parasites. In response to antigen recogni-
markers and perform different functions in the immune tion and assistance from T helper lymphocytes, naive B
response. The B lymphocytes, the producers of antibod- lymphocytes will proliferate and differentiate into plasma
ies, or the humoral response, are critical for defense cells, which secrete antibody and are relatively short‐
against extracellular pathogens. The cytotoxic T lym- lived, or memory cells, which are long‐lived cells that are
phocytes, which force altered and invaded cells to more easily activated and respond more quickly and with
undergo apoptosis, are critical for defense against intra- higher magnitude and antibody affinity than naive B
cellular pathogens. The overseers of the entire response, lymphocytes. Producing a significant population of the
T helper lymphocytes, direct and modulate the response latter is the goal of most vaccines. Antibodies, the effec-
using cell surface molecule expression and cytokine tor mechanism of B lymphocytes, will opsonize, neutral-
production and secretion. It is important to understand ize, clump for easy removal of, or activate complement to
how various lymphocyte lineages recognize antigens in destroy the infecting agent.

Clinical Small Animal Internal Medicine Volume II, First Edition. Edited by David S. Bruyette.
© 2020 John Wiley & Sons, Inc. Published 2020 by John Wiley & Sons, Inc.
Companion website: www.wiley.com/go/bruyette/clinical