Immunity

Definition

 

Immune system produces antibodies against antigens

 

Immune system remembers

- second exposure to antigen generates greater & more rapid response

 

Two types of immunity

- humoral

- cell mediated

 

Two types immunization

- active

- passive

 

Development

 

Lymphocyte precursors come from bone marrow

- migrate to modifying organ

- then return to nodes & bone marrow

 

T Cells

 

Lymphocytes that populate thymus

 

Mediate cellular immunity

 

Differentiate into 4 types

1.  Helper / Inducer T Cells (CD4)

2.  Suppressor T Cells (CD8)

3.  Killer T Cells (CD8)

4.  Memory T Cells

 

Helper T Cells

- involved with regulation of antibody production by B cells

 

Killer cells

- destroy foreign cells

 

B Cells

 

Cells that populate spleen & liver

 

Mediate Humeral Immunity

 

B lymphocytes differentiate into

- pasma cells

- memory B cells

 

Humoral Immunity

 

Basic Process

 

Foreign proteins that enter body called antigens

- ingested by macrophages

- macrophages expose part of ingested antigen plus Major Histocompatibility Complex (MHC) on surfaces

 

Macrophages then contact lymphocytes

- CD4 (TH) cells bind to Antigen + MHC II on surface of macrophage

- CD4 cells become activated & contact B cells

- B cells activated

- proliferate

- transform into Memory B cells & plasma cells which secrete antibodies

 

Immunoglobulins

 

5 types

- shaped like Y

- stem is Fc Fragment

- arms are Fab Fragment

 

Consist of

1. Two light chains

- two types

- Kappa & Lambda

 

2. Two heavy chains

 

IgG

- Commonest

- Function is Complement fixation

- cross Placenta

 

IgA

- Function is localised protection secretions

 

IgM

- function is complement fixation

- immature Ig

- largest of the five

- can't cross placenta

 

IgD

- function is antigen recognition by B cells

 

IgE

- function is Reagin activity

- releases Histamine from mast cells

 

Major Histocompatibility Complex

 

Located on short arm of Chromosome 6

- code Glycoproteins located on surfaces of all cells

- function in distinguishing self from non self

- three major loci

 

Class I Loci

- encode for HLA-A, HLA-B, HLA-C

- found on all cells except RBC

- primary factors in Self-recognition & Development of tolerance

 

Class II Loci

- encode for HLA-DP, HLA-DQ, HLA-DR

- involved in Antigen presentation to Helper T cells

- present on Antigen Presenting Cells

- macrophages, B Cells, activated T cells

 

Class III Loci

- encode for proteins of complement cascade

 

Complement System

 

When antigens combine with circulating Ig

- cells lysed

- bacteria opsonised (phagocytosed)

- WCC attracted to antigen

- histamine released

 

Mediated by plasma enzymes called complement

- enzymes numbered C1 to C9

- C1 binds to Ig that have bound antigen

- triggers series of events that activates other components of system

- C3 facilitates phagocytosis of micro-organisms

- C5-9 inserted into cell membranes as pores

- C5a & C9a cause Histamine release from mast cells

- C5a is chemotactic for neutrophils

 

Cellular Immunity

 

Mediated by CD8 cells

- activated when presented with antigen & MHC-I protein

 

Proliferate & differentiate into

- Memory T cells

- Cytotoxic T cells

- Suppressor T cells

 

Cytotoxic cells kill by

1.  Insertion of pore-forming protein into target’s cell membranes

2.  Insertion of toxins into target cells

 

Suppressor cells help to terminate immune response

 

Memory cells responsible for accelerated response to second exposure