Immune response to bacterial infections
The immune response to bacterial infections is a complex and coordinated defense mechanism employed by the body to protect against pathogens. It begins with physical barriers, such as the skin and mucous membranes, which prevent bacteria from entering the body. When bacteria breach these barriers, the immune system activates various cells, including white blood cells known as leukocytes, to identify and eliminate the invaders. There are two primary types of immunity involved: antibody-mediated (humoral) immunity, which relies on antibodies produced by B cells, and cell-mediated immunity, which involves T lymphocytes that target and destroy infected cells.
The immune response can be categorized into primary and secondary responses. The primary response occurs upon first exposure to a specific antigen, leading to a slower but essential buildup of antibodies. In contrast, the secondary response is much quicker and more robust due to the presence of memory B and T cells that rapidly recognize and respond to previously encountered antigens. This efficient system not only helps combat current infections but also contributes to long-term immunity, reducing the likelihood of reinfection. Overall, the immune response evolves as individuals age and gain exposure to various pathogens, enhancing their ability to fend off bacterial infections.
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 and 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 encounters 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 the 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 the 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 antigens 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. Interleukin-7 is also important in maintaining the survival, proliferation, and functioning of the body’s T cell population.
Some research indicates that macrophages and other innate immune cells like monocytes form a recognition of infections that allows them to respond more efficiently the next time the body contracts a pathogen with similar characteristics. This is called innate immune memory or trained immunity and occurs through cell epigenetic changes. Unlike adaptive immunity, this memory is not antigen-specific but improves the body’s ability to respond to new threats.
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 throughout a person’s life. Adults and teenagers generally get fewer bacterial infections than younger children because their bodies have learned to recognize and immediately attack the antigens to which they are exposed.
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