Chapter summary

Introduction

The immune system, through both nonspecific (innate) and specific (humoral and cellular) pathways, acts to prevent infections and fight infections when they do occur.

Specific immunity

Two key features of specific immunity are (1) that defense is "engineered" to attack and destroy a par­ticular bacterium or agent and (2) that responses have an aspect of memory. While it is convenient to discuss cellular immunity, that is, the actions of lym­phocytes (T cells, B cells), macrophages, neutro­phils, and dendritic cells as independent of humoral immunity (production and circulation of antibodies), it is important to appreciate that functions occur in concert. In other words, the effectiveness of either depends on the other.

Antibodies

There are multiple classes of antibodies (IgA, IgG, etc.), but these immunoglobulins are produced and secreted by B cells that have been stimulated by anti­gens in connection with other lymphocytes (T cells or macrophages) to first divide and produce daugh­ter cells. Some of these daughter cells azre further stimulated to differentiate into plasma cells. The plasma cells are essentially miniature antibody-making fac­tories. This explains the increase in antibodies in cir­culation in the days and weeks following exposure to a foreign antigen. An antigen, by the way, is a protein, polysaccharide, or other molecule that is capable of eliciting the proliferation of B lympho­cytes and the differentiation to create plasma cells. Some of the B cells that were initially stimulated (the population of daughter cells) can subsequently divide and produce clones of cells, but these clones retain the capacity to very rapidly respond if they are exposed to the same antigen a second time.

Leukocytes, lymphocytes, and others

The distinction between all of the various white blood cells can be confusing, but to understand the basics of immunity, it is important to have some appreciation of these various cells. Lymphocytes are divided into two broad classes: T cells and B cells. In mammals, the T cells get their name because during development, the initial population of T cells is pro­duced in the thymus. Because of subsequent stimula­tion and migration, these cells further differentiate to become helper T cells (T_h), killer or cytotoxic T cells (T_C), or suppressor T cells (T_s). Each of these classes of T cells is identified based on the array of surface markers that they express. As the names suggest, T_c cells with proper stimulation are capable of destroy­ing other cells, that is, cells infected by a virus. The T_h cells act by assisting or helping the actions of B cells to differentiate or other T cells to become acti­vated. The T_s cells function as regulators to prevent excessive immunity responses. B lymphocytes (B cells) originally got their name because they were identified in lymphoid tissue called the bursa of Fabricius, which is located near the cloaca of chick­ens. These are the cells that give rise to other B cells with "memory" as well as the plasma cells that syn­thesize and secrete immunoglobulins.

Cytokines

Control and regulation of most immune responses depends on the secretion of powerful hormone-like molecules or messengers collectively called cyto­kines or immunokines. These proteins are typically secreted by the various leukocytes. Their targets are typically other lymphocytes and related cells (macro­phages, dendritic cells, neutrophils). Examples include an entire suite of 36 different interleukins, interfer­ons, TNF-α, and many others.

Passive immunity

This refers to the protection that is provided by the administration of antibodies to a patient or more often the passage of antibodies from mother to off­spring in utero (depending on the type of placenta­tion). For example, in ruminants there is no in utero transfer of immunoglobulins so antibodies received from the sucking or feeding of colostrum is essential to provide some immunity until the calf, kid, or lamb can begin to generate its own specific immunity.

Lymphatic system

This is a system of thin vessels and interconnected series of lymph nodes and related organs, such as spleen, tonsils, thymus, Iacteals, and nodules, which provide a pass for interstitial fluid to return to the general circulation in the form of lymph that empties at the thoracic duct. The lymph nodes and related organs provide sites for the proliferation and con­centration of lymphocytes that can ultimately be recruited to enter the circulation or tissue regions in response to inflammation.

Review questions and answers are available 3 online.

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