What is the immune response to bacterial infections?

Definition

The immune system defends itself against infectious organisms
(pathogens) such as bacteria by
utilizing physical barriers that prevent bacteria from entering the body and by
detecting and eliminating bacteria after they enter the body. Cells, proteins,
tissues, and organs work together in a coordinated response, the immune response,
to defend against microorganisms.

When a bacterial infection develops, the immune system responds
through a series of steps by activating certain cells and by producing substances
that recognize and react to invading microorganisms, or antigens.
Bacterial antigens are generally proteins present on the surface of a
bacterium.

Types of Immunity

Physical barriers are the immune system’s first line of defense. They comprise
the skin, mucous membranes, mucus, and tears. Unless damaged through injury or
other means, the skin generally protects against invasion by microorganisms.
Mucous membranes (that is, the linings of the mouth, nose, and eyelids) are
effective barriers and are generally coated with secretions, such as lysozyme,
that fight microorganisms. Organisms that penetrate physical barriers are
identified and eliminated by white blood cells and antibodies. Adaptive immunity, comprised of cell-mediated and antibody-mediated
immunity, is an important component of defense against bacterial infection. In
antibody-mediated, or humoral, immunity, the immune response is mediated by
antibodies (immunoglobulins), which are specific proteins produced in response to
antigens. Cell-mediated immunity is mediated by effector T cells (T
lymphocytes).

Cells Involved in an Immune Response

The immune system is made up of a coordinated network of cells, tissues, and
organs. White blood cells, or leukocytes, circulate and detect and destroy
microbes. Two basic types of leukocytes are phagocytes and lymphocytes. Phagocytes ingest invading organisms and
lymphocytes help recognize invaders and eliminate them. The neutrophil is the most common type of phagocyte and is
primarily involved in fighting bacteria; an increase in neutrophil numbers
generally is triggered by infection. Leukocytes circulate in the bloodstream to
provide a coordinated effort for the immune system to monitor and protect against
bacterial infection.

B and T lymphocytes (B and T cells) have separate functions: B cells seek targets and send defenses and T cells, in various forms, destroy the invading organism. With stimulation by antigens, T cells comprise several forms, or classes, of effector T cells: killer (cytotoxic), helper, and suppressor.

Killer T cells destroy specific target cells. Helper T cells help other cells, such as B cells, produce antibodies;
they also help activated killer T cells destroy foreign cells (macrophages), which
enables the killer T cells to ingest foreign cells efficiently. T cells also
produce cytokines that activate other cells. B cells have receptors on their
surface, where antigens attach stimulate cells to become antibody-secreting
cells.

Primary and Secondary Immune Response

A primary immune response occurs the first time antigens are encountered. At subsequent encounter with the same antigens, a secondary immune response occurs. Before an infection, precursor T or B cells are present as resting cells, but during the course of an adaptive immune response, the immune system activates T cells or triggers B lymphocytes to produce antibodies. After initially encountering an antigen, sufficient amounts of antibody take several days to produce, with only small amounts formed during the first few days; circulating antibodies are undetectable until about one week after initial encounter. The primary immune response is relatively slow, with antigens first needing to be recognized, processed, and presented by antigen-presenting cells. Antibody levels need to reach sufficient levels for the host to develop resistance (which may take several days or weeks).

A second encounter with microbial antigens leads to an accelerated immune response, called the secondary or memory response. During the secondary response, memory B cells “remember” and rapidly recognize antigens. Memory B cells then multiply and change into plasma cells; large amounts of antibodies are generated in only one to two days. Similarly, memory T cells rapidly develop into effector cells. The secondary immune response is very quick, efficient, and effective. This specific immune response prevents people from contracting certain diseases more than once.

Antigen-Presenting Cells

The primary immune response is initiated when an antigen penetrates epithelial
surfaces and comes into contact with macrophages or other antigen-presenting
cells. An antigen-presenting cell is usually either a macrophage or a dendritic
cell and, in combination with either a B or T cell, is required for an immune
response. Antigens, such as bacterial cells, are ingested and processed by
antigen-presenting cells and then presented to lymphocytes to initiate the immune
response.

Processing by a macrophage results in antigen fragments being attached in combination with cell surface molecules known as MHC. The antigen-MHC complex is presented to helper T cells, which recognize processed antigen and develop into effector T cells. When a macrophage presents antigen to a B cell, the B cell is signaled to generate antibodies specific for that antigen.

Cell Signaling

Helper T cells provide signals, such as interleukins or cytokines, that stimulate cells to proliferate and function more efficiently. The interaction between an antigen-presenting macrophage and a helper T cell results in secretion of interleukin-1 from macrophages that, in turn, stimulate helper T cells to mature and produce other cytokines, including interleukin-2 and -4. Interleukin-2 stimulates proliferation of other T cells and interleukin-4 causes B cells to develop into antibody-secreting plasma cells. Interleukin-2 also activates killer T cells to destroy cells with antigen on their surfaces. When a B cell is stimulated by interleukin-4, the B cell grows and divides to form an army of identical B cells, each capable of producing large amounts of identical antibody molecules.

Impact

The immune system prevents and defends against bacterial infections. Antibody-mediated and cell-mediated immune responses are generated during almost all infections, but the magnitude and importance of each response varies, depending on the host and the infectious agent. As people age, they usually become immune to more microorganisms, as the immune system comes into contact with increasing numbers of antigens through a person’s life. In general, adults and teenagers tend to get fewer bacterial infections than younger children because their bodies have learned to recognize and immediately attack antigens to which they are exposed.

Bibliography

Coico, Richard, and Geoffrey Sunshine. Immunology: A Short Course. 6th ed. Hoboken, N.J.: Wiley-Blackwell, 2009. A clear and comprehensive introduction to essential topics in modern immunology.

DeFranco, Anthony L., Richard M. Locksley, and Miranda Robertson. Immunity: The Immune Response in Infectious and Inflammatory Disease. New York: Oxford University Press, 2007. An introduction for undergraduates and medical students to the immune response to infection. Includes chapters on the immune response to specific microorganisms.

Mak, Tak W., and Mary E. Saunders. Primer to the Immune Response. Maryland Heights, Mo.: Academic Press/Elsevier, 2008. Provides an understandable introduction to immunology. A resource for college students, students studying medicine, and those in health professions.