Basic immunology notes taken from Mims microbiology. Mims' Medical Microbiology and Immunology, 6th Edition. Summarised immunology basics split into 6 chapters.
IMMUNOLOGY BASICS FOR BEGINNERS
Chapter 1
The physiologic function of the immune system is defence against infectious microbes.
However, even non-infectious foreign substances can elicit immune responses.
Defence against microbes is mediated by the early reactions of innate immunity and the later
responses of adaptive immunity.
Innate immunity
- early line of defence. Cellular and biochemical defence mechanisms. Mechanisms react to
products of microbes and injured cells, and respond in essentially the same way to repeated
exposures. - principal components are physical and chemical barriers (1) and phagocytic
cells, DCs and NK cells (2) and blood proteins (3).
Adaptive immunity
- develops as a response to infection and adapts to the infection
- ability to distinguish different substances, called specificity and the ability to respond more
vigorously to repeated exposures to the same microbe, memory.
- Lymphocytes that secrete antibodies. Antibodies recognize antigens.
- Cytokines are a large group of secreted proteins with diverse structures and functions, which
regulate and coordinate many activities of the cells of innate and adaptive immunity.
Interleukin. A large subset of structurally related cytokines that regulate cell migration and
movement are chemokines.
Two types of adaptive immune responses:
1. Humoral immunity. Mediated by molecules in the blood and antibodies that are
produced by B lymphocytes. Principal defence mechanism against extracellular microbes and
their toxins, because secreted antibodies can bind to these microbes and toxins and assist in
their elimination.
2. Cell-mediated immunity. Mediated by T lymphocytes. Promotes the destruction of
microbe residing in phagocytes or the killing of infected cells to eliminate reservoirs of
infection.
Active immunity: the form of immunity that is induced by exposure to a foreign antigen. The
immunized individual plays an active role in responding to the antigen.
Naïve lymphocytes are immunologically inexperienced.
Passive immunity: becoming immune to the particular antigen without ever having been
exposed to or having responded to that antigen. Useful method for conferring resistance
rapidly, without having to wait for an active immune response to develop. Ex. Antibodies
transferred through placenta to fetus.
Immunogen: substance that stimulate immune responses.
Opsonization: factors in immune serum enhance phagocytosis of bacteria by coating the
bacteria.
,A reaction to an antigen is detectable only in individuals who have previously encountered
the antigen; these individuals are said to be sensitized to the antigen, and the reaction is an
indication of sensitivity.
Epitope: the part of an antigen that is specifically recognized by individual lymphocytes.
The ability of the lymphocyte repertoire to recognize a very large number of antigens is the
results of variability in the structures of the antigen-binding sites of lymphocyte receptors for
antigens = diversity.
Specificity and diversity
memory
clonal expansion
specialization
contraction and homeostasis
nonreactivity to self.
Immunologic unresponsiveness = tolerance. Tolerance to self-antigens (self-tolerance) is
maintained by different mechanisms. These include eliminating lymphocytes that express
receptors specific for some self-antigens, inactivating self-reactive lymphocytes, or
suppressing these cells by the actions of other cells. Abnormalities of self-tolerance may lead
to autoimmune diseases.
The principal cells of the adaptive immune system are lymphocytes, antigen-presenting cells
and effector cells.
B lymphocytes are the only cells capable of producing antibodies. They recognize
extracellular soluble and cell surface antigens, and they differentiate into antibody-secreting
plasma cells, thus functioning as the mediators of humoral immunity.
T lymphocytes, the cells of cell-mediated immunity, recognize the antigens of intracellular
microbes and the T cells either help phagocytes to destroy these microbes or they kill the
infected cells. T lymphocytes have a restricted specificity for antigens; they recognize
peptides derived from foreign proteins that are bound to host proteins called major
histocompatibility complex (MHC) molecules, which are expressed on the surface of other
cells. The T cells recognize and respond to cell surfaceassociated but not soluble antigens.
- helper T cell (Th). Secrete cytokines and these stimulate the proliferation and
differentiation of the T cells themselves and activate other cells (B cells, macrophages etc). -
cytotoxic T lymphocytes (CTL). Kill cells that produce foreign antigens - regulatory T cells
(Treg). Inhibit immune response.
- NKT cells. Small population of T lymphocytes that express some cell surface proteins
found on NK cells.
, APC: capture and display antigens to specific lymphocytes. This is required to initiate and
develop adaptive immune responses. DCs are the most specialized APCs, which capture
microbial antigens that enter from the external environment, transport these antigens to
lymphoid organs, and present the antigens to naïve T lymphocytes to initiate immune
responses.
Effector cells: required for antigen elimination. Mediate the final effect of the immune
response, which is to get rid of microbes.
The innate immune system blocks the entry of microbes and eliminates or limits the growth
of many microbes that are able to colonize tissues. The cellular innate immune response to
microbes consists of two main types of reactions: inflammation and antiviral defense.
Inflammation: the process of recruitment of leukocytes and plasma proteins from the blood,
their accumulation in tissues, and their activation to destroy the microbes. Phagocytes,
neutrophils and monocytes are the major leukocytes that are required.
Antiviral defence consists of a cytokine-mediated reaction in which cells acquire resistance to
viral infection and killing of virus infected cells by NK cells.
Because many pathogenic microbes have evolved to resist innate immunity, adaptive
immunity is necessary.
The adaptive immune system uses three main strategies to combat microbes:
1. Antibodies. Secreted antibodies bind to extracellular microbes, block their ability to
infect host cells, and promote their ingestion and subsequent destruction by phagocytes.
2. Phagocytosis. Phagocytes ingest microbes and kill them, and antibodies and Th cells
enhance the microbicidal abilities of the phagocytes.
3. Cell killing. CTLs destroy cells infected by microbes that are inaccessible to
antibodies and phagocytic destruction.
Clonal selection hypothesis: the fundamental concept that lymphocytes specific for a large
number of antigens exist before exposure to the antigen, and when an antigen enters a
secondary lymphoid organ, it binds to (selects) the antigen-specific cells and activates them.
According to this hypothesis, antigen-specific clones of lymphocytes develop before and
independent of exposures to antigen. A clone refers to a lymphocyte of one specificity and its
progeny. A characteristic of the immune system is that a very large number of clones is
generated during the maturation of lymphocytes of clones is generated during the maturation
of lymphocytes, thus maximizing the potential for recognizing diverse microbes.
Because T cell receptors are specific for MHC-associated peptides, these lymphocytes can
interact only with cell-associated antigens (because MHC molecules are cell surface proteins)
and not with free antigen.
All the functions of T lymphocytes are dependent on their physical interactions with other
cells. To respond, the T cells need to recognize not only antigens but also other molecules,
costimulators, which are induced on the APCs by microbes.
Activated CD4+ Th lymphocytes proliferate and differentiate into effector cells whose
functions are mediated largely by secreted cytokines. When naïve CD4+ T cells are activated
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