Basic Concepts and components of the Immune System

Toll-like receptors (TLRs)

are one type of pattern recognition molecule used by innate immune system cells. Present on macrophages. TLRs recognize a variety of substances found on a range of microbes but not on host cells, for example, they bind bacterial LPS.

Mannan-binding lectin (MBL)

is a type of pattern-recognition molecule, present in plasma in solution and used by the complement system. MBL recognizes sugar molecules containing mannose on the surface of bacteria, fungi, and viruses and helps activate the complement cascade. Mannan is also present on a the surface of damaged cells. Thus MBL recognizes and binds both microbes and damaged host cells but not normal cells.

NK cells

one of the components of the innate IS. They can detect and kill certain infected cells by inducing programmed cell death (apoptosis) and by the production of cytokines.

Interferons

Are part of the cytokine family, which is a part of the innate IS.

A viral infection triggers interferon production by the infected cell. Interferons inhibit the replication of many viruses and are not pathogen-specific.

3 major features of the adaptive IS

1. Specificity

2. Diversity

3. Memory

Specificity

ability to discriminate among various molecular entities

Diversity

Ability to respond to almost any antigen that may be encountered

Memory

Ability to recall previous contact with antigen and show a stronger response to the second time.

Elimination of extracellular pathogens

Occurs chiefly by antibodies

Elimination intracellular pathogens

Requires macrophages and T-cells (sometimes the function of T-cells is called cell-mediated immunity)

Active immunity

An individual plays a direct role in responding to the antigen (after an encounter with an organism)

Passive immunity

Immunity is transferred from one individual to another by transferring immune cells or serum from an immunized individual to an unimmunized individual

Phases of an immune response

1. Cognitive phase (antigen is recognized)

2. Activation phase (more and more of the same clone of cells is produced - proliferation)

3. Effector phase: to enable a response cells undergo various changes (differentiation) - e.g. B-cells become plasma cells

Types of immune response

1. Intracellular pathogens - MBT, viruses, protozoa. Innate IS: interferon, macrophages, NK cells. Adaptive IS: TH1 T cells, Cytotoxic T lymphocytes IgM, IgG, IgA

2. Surface pathogens: helminths, arthropods. Innate IS: mast cells, eosinophils. Adaptive IS: TH2 T cells, IgE

3. Extracellular: bacteria, fungi. Innate IS: neutrophils. Adaptive IS: TH17 T cells