Host Defenses and Immunization

Comprehensive flashcards covering host defenses, the components of the innate and adaptive immune systems, antibody classes, and principles of immunization based on the lecture material.

First line of defense

Any barrier that blocks invasion at the portal of entry and limits access to internal tissues; characterized as general in action and not a true immune response.

Second line of defense

An internalized system of protective cells and fluids, including inflammation and phagocytosis, that acts rapidly at local and systemic levels.

Third line of defense

Specific immunity acquired as foreign substances are encountered by lymphocytes; produces unique substances for each microbe and provides long-term immunity.

Lysozyme

An enzyme found in tears, saliva, blood, and phagocytes that degrades peptioglycan.

Peroxidase

An enzyme found in saliva, body tissues, and phagocytes that breaks down hydrogen peroxide to produce reactive oxygen.

Lactoferrin

An iron-sequestering protein found in saliva, blood, and tissue fluids that inhibits microbial growth by making iron unavailable.

Defensins

Antimicrobial peptides found on mucous membranes and in phagocytes that insert into microbial membranes.

Normal flora

Microorganisms found on body surfaces of healthy individuals that provide protection through competitive exclusion for binding sites and nutrients.

Hematopoiesis

The process of blood cell formation where all blood cells, including immune cells, originate from hematopoietic stem cells in the bone marrow.

Neutrophils

The most abundant and important granulocytes in the innate response, also known as polymorphonuclear neutrophilic leukocytes (PMNs).

Basophils

A type of granulocyte involved in allergic reactions.

Eosinophils

Granulocytes primarily involved in expelling parasitic worms and active in allergic reactions.

Monocytes

Phagocytic leukocytes that circulate in the blood and differentiate into macrophages when they enter tissues.

Dendritic cells

Branched cells that act as scouts in tissues, engulfing material to present it to cells of the adaptive immune system.

B cells

Lymphocytes responsible for humoral immunity; they produce specialized plasma cells which secrete antibodies.

T cells

Lymphocytes responsible for cell-mediated immunity; they modulate immune functions and kill foreign cells.

Markers

Proteins and/or sugars on cell surfaces that allow the immune system to identify whether a cell is self or a nonself threat.

Pathogen-associated molecular patterns (PAMPs)

Markers that many different kinds of microbes have in common.

Pattern Recognition Receptors (PRRs)

Receptors used by host cells in the second line of defense to recognize PAMPs.

Complement system

A series of nine proteins ($C1$–$C9$) that circulate in blood in inactive form and initiate a cascade for inflammation, opsonization, and lysis.

Alternative pathway

A complement activation pathway triggered by the binding of complement protein $C3b$ to a cell surface.

Lectin pathway

A complement activation pathway requiring mannan-binding lectins (MBL) to detect mannan on microbial cells.

Classical pathway

A complement activation pathway that requires the interaction of antibodies with complement $C1$.

Membrane attack complex (MAC)

A donut-shaped structure formed by $C5b$, $C6$, $C7$, $C8$, and $C9$ that creates pores in Gram-negative membranes to cause lysis.

Diapedesis

The process by which phagocytes leak out of blood vessels into tissues during inflammation.

Pyrogens

Fever-inducing substances that reset the hypothalamic thermostat; categorized as endogenous (cytokines) or exogenous (microbial products).

Immunogen

A term used for any antigen that specifically elicits an immune response.

Epitope

The specific region of an antigen to which the immune response is directed; also called an antigenic determinant.

Fab region

The unique variable region of an antibody that binds to a specific antigen.

Fc region

The constant region of an antibody's heavy and light chains.

IgM

The first antibody class produced in response to infection; it is a pentamer structure and constitutes $5$–$13\%$ of circulating antibodies.

IgG

The most prevalent antibody in circulation ($80$–$85\%$); it is the only class that can cross the placenta and serves as the antibody of memory.

IgA

The antibody class found in secretions such as mucous, tears, saliva, and breast milk; exists as a dimer in secretions.

Antigen presenting cells (APC)

Cells, such as macrophages, that destroy antigens and present portions of them on their surface next to self antigens.

Affinity maturation

A form of natural selection among proliferating B cells that improves the quality of the immune response by making receptors more specific to the antigen.

Class switching

The process where B cells switch from producing $IgM$ to other classes of antibody, such as $IgG$, under the influence of helper T cell cytokines.

MHC class I

Molecules that bind endogenous antigens and are recognized by Cytotoxic T cells ($CD8$).

MHC class II

Molecules that bind exogenous antigens and are recognized by Helper T cells ($CD4$).

Natural killer (NK) cells

Lymphoid cells that lack antigen specificity and can recognize antibody-coated cells or host cells with no MHC class I molecules.

Active immunity

Immunity resulting from an individual's own immune response following exposure to an antigen through illness or immunization.

Passive immunity

Immunity resulting from the transfer of antibodies produced by another person or animal, such as through the placenta or breastfeeding.

Toxoid vaccines

Vaccines that use a toxin made by a germ to generate immunity against the product of the germ rather than the germ itself.

Titer

The concentration of antibody in serum, which serves as an indicator of previous exposure.