Immune system text chapter 18

1
Immune system (text chapter 18)

• Cells, tissues, and molecules involved in defense
  against viruses, bacteria, and parasites
• Terminology of mechanisms/systems
• Innate vs adaptive
• Non-specific vs specific
• Cellular vs humoral

2
Components of immune system

• Cells of 10 types
• in blood, lymphatic system, and tissues
• Include lymphocytes (T B cells)
• Molecules
• Immunoglobulins (antibodies) and T-cell receptors
• Other defensive molecules, e.g. complement,
  perforins
• Cytokine hormones, including interferons

3
Blood vascular and lymphatic systems

• Blood vascular system
• Heart blood vessels
• blood
• Lymphatic system
• Lymphatic vessels
• lymph nodes
• lymph
• extracellular fluid

4
Adaptive immunity (18.3)

• Antigens are molecules recognized by the immune
  system as non-self
• Recognition involves binding by proteins called
  immunoglobulins
• Immunoglobins
• produced by lymphocytes
• antibodies and receptors

5
Lymphocytes recognize antigens

• B-cells
• Mature in bone marrow
• Produce humoral response (antibodies)
• Become plasma cells, memory cells
• T-cells
• Mature in thymus
• Produce cellular responses, via cytokines
• Helper (CD4) T-cells, and cytotoxic T-cells

6
Questions re adaptive immunity

• How can the system recognize so many kinds of
  foreign molecules so specifically?
• How does the system distinguish self molecules
  from non-self?
• How does immunity develop and persist?

7
Immunoglobulin Structure (18.9)

• Multimeric protein
• Each has at least two variable regions that
  bind antigen and a...
• Constant region

8
Somatic recombination (see 18.6)

• The genes for the variable regions of
  immunoglobulins (and T-cell receptors)recombine
  during cell development and maturation
• This process of somatic recombination generates a
  library of T-cells, each with unique variants
  of the recombinant genes
• If a matching antigen is encountered, the cell is
  activated to multiply

9
Clonal selection(18.6)

• Somatic recombination during development
  generates a large number of genetically unique
  lymphocytes, each producing particular Ig variant
• Activation by antigen binding Ig results in a
  clone of effector and memory cells that produce
  the relevant Ig

10
B-cells and immunoglobulins (Ig)

• 5 classes of Ig (table 18.3) each with particular
  functions
• IgD acts as receptor on surface of B-cells during
  activation
• IgG is the most abundant circulating (humoral) Ig
  class
• IgE triggers histamine release, important in
  allergic reactions
• Classes differ in constant regions

11
Antibody functions

• Two hands so can clump particles bearing
  antigen, e.g. bacteria or viruses.
• Promote phagocytosis by macrophages.
• Trigger complement cascade to lyse cells bearing
  antigen (this is a complex pathway involving
  multiple kinds of enzymes)
• Monoclonal antibodies produced by cell cultures
  are used for many diagnostic purposes, e.g.
  pregnancy tests.

12
Mechanism for allergy (18.19)
Antigen triggers B-cell IgE production IgE
binds mast cell receptors More antigen
triggershistamine release
13
T-cell receptors

• T-cells receptorsbind antigen
• Like antibodies somatic recombination produces
  unique variants
• Binding of antigen activates T-cell to divide,
  produce clone

14
Major histocompatibility complex

• The MHC proteins are glycoproteins that bind
  intracellular antigen, move to the cell membrane,
  and present antigens to T-cells
• T-cells dont bind antigen directly- they bind
  antigen fragments bound to MHC proteins
• MHC proteins present short peptide fragments
  (10-20 aa peptides) from partially degraded
  proteins (diagram).

15
Antigen-presenting cells T-cells do not receive
antigens directly- antigen fragments are
presented by macrophages (18.13-18.15)
16
T-cell responses to antigen

• Cytotoxic T-cells kill virus-infected somatic
  cells (recognized by MHC-antigen complexes)
• Helper T-cells (TH cells) produce endocrines
  (cytokines) that stimulate other immune
  processes, such as complement cascade,
  multiplication of activated B-cells, and others

17
MHC polymorphism

• MHC region is family of 140 genes on chromosome 6
  in humans
• The MHC genes are highly polymorphic- some of
  them have hundreds of different alleles in the
  human population.
• MHC proteins on cell surface can act themselves
  as antigens and determine histocompatibility

18
Recognition of self vs nonself

• Developing lymphocytes (B-cells and T-cells) each
  have unique receptors for antigen
• During development, cells with receptors that
  bind self molecules die.
• This developmental process culls lymphocytes that
  would otherwise cause autoimmune disease

19
MHC mate choice

• Greater variety of MHC alleles presumably
  provides more effective immune protection.
• Animals (including humans) tend to choose mates
  that have MHC alleles different from their own.
• Discrimination is olfactory
• http//www.pbs.org/wgbh/evolution/library/01/6/l_0
  16_08.html