Immunology in a Nutshell

• Innate Immunity – Rapid responses to a broad range of microbes
• Acquired Immunity – Slower responses to specific microbes

External Defence (First line of defence)	Internal Defence (Second line of defence)	Internal Defence (Third line of defence)
Skin	Phagocytic cells	Humoral response (antibodies)
Mucosa membrane	Antimicrobial proteins	Cell-mediated response
Secretions	Natural killer cells	Cytotoxic lymphocytes

FIRST LINE OF DEFENSE: (Aim) Prevent microorganisms from gaining access to the body.

There are different types of natural barriers that prevent the entry of foreign bodies inside. Some of the natural barriers include skin, mucosa membrane and secretions as well.

SECOND LINE OF DEFENSE: (Aim) To recognize, neutralize and destroy invaders.

The body's way of dealing with non-specific invasion

 

Acquired Immunity: (Pathogen Specific):

1^st time attack – mild response by the body = Primary response

2^nd time attack – more intense response = Secondary response or ANAMNESTIC RESPONSE

Third line of defense: specific defense mechanisms

Q. How do we deal with specific and recurring invasion?

Lymphocytes are pivotal cells to deal with specific defence mechanisms. Primary and secondary responses are carried out with the help of two specific lymphocytes – B lymphocytes and T-lymphocytes.

§  B-lymphocytes produce an array of protein in response to pathogens into our blood to fight with them. These proteins are called ANTIBODIES.

  Each antibody is a protein molecule called Immunoglobulin.
Antigens are usually glycoproteins. 


Antigens are usually glycoproteins. 

Q. Where they come from? Where do they live?

Q. How do we know that lymphocytes are required for adaptive immunity?

Q. How do we indicate the specific response and get the lymphocytes involved?

Foreign antigens are presented for identification, but first we need to distinguish “self” from “foreign.”

Two classes of proteins:

• Class I MHC – present on all nucleated cells
• Class II MHC – present on macrophages, dendritic cells and B-cells

 

The macrophage represents the antigen to the outside world that a foreign body has been recognized. The MHC-antigen complex is first recognized by T-helper cells.

T-helper cells when meet with APC, they get activated and secrete cytokines to stimulate B-cells to produce antibodies. This complex is further supported by CD₄ molecule.

Antigen recognition by T₄ leads to a great activation process. In the meantime, any infected cell (eg. Muscle cell) might be infected by the same bacterium.

 

The T-lymphocytes do not secrete antibodies, but help B-cells produce them.

Each antibody consists of:

• 4 peptide chains –
• 2 small (light chain)
• 2 longer (heavy chain)

Hence, an antibody can be represented as H₂L₂

Types of antibodies:

(NOTE: USE THE WORD MEGA-D TO REMEMBER THEM)

• I_gM = first antibody to be produced in the body in response to antigens 5 H₂L₂
• I_gE = for allergy, 1 H₂L₂
• I_gG = 80% of the serum antibodies 1 H₂L₂; can cross the placenta
• I_gA = 2 H₂L₂
• I_gD = 1 H₂L₂

 

Note: The yellowish fluid secreted by mother during the initial days of lactation has abundant antibodies (I_gA) to protect the infant = PASSIVE IMMUNITY

ACTIVE IMMUNITY = antibodies are produced in response to the antigen in the body (slow process).

PASSIVE IMMUNITY = Injection of antibody to patients directly.