States of Immunity
Types of IG's

 I. INNATE - called natural, native, genetic, constitutional;
	 based on physiological, anatomical, and biochemical 
	 differences

	 A. SPECIES - many microbes can infect only certain species
	 B. RACIAL - certain classes or ethnic stocks of a species
	    are resistant or susceptible to given infectious agents
	 C. INDIVIDUAL - individuals having differing levels of
	    resistance or susceptibility; differences are due to 
	    factors such as: genetic defects, age, hormonal balance,
	    nutritional status, and unexplained unknown factors

  II. ACQUIRED IMMUNITY - based on responses by the immune system

	 A. ACTIVE - body is stimulated to produce antibodies as a 
	    result of microbial challenge or insult; provides long
	    term protection; antibodies (Ab) and sensitized T cells
	    are produced in the process and continue to circulate in
	    the blood and lymph

	    1. ARTIFICIALLY ACQUIRED, ACTIVE - follows exposure to 
	       prepared antigens such as toxoids, killed or inacti-
		  vated microorganisms or attenuated microorganisms
	    2. NATURALLY ACQUIRED, ACTIVE - follows infection that 
		  has been acquired in day-to-day activities

	 B. PASSIVE - the body receives antibodies or immune product
	    from some outside source; thus the body is NOT stimu-
	    lated to produce its own antibodies; this type of pro-
	    tection is of short duration (until the acquired Ab are
	    broken down or used up)

	    1. ARTIFICIALLY ACQUIRED, PASSIVE - follows the injec-
	    	  tion of immune products that were obtained from 
		  another species or from members of the same species
	    2. NATURALLY ACQUIRED, PASSIVE - results from the trans-
	   	  fer of immune products from one individual to another
		  under natural circumstances (transplacental transfer)

Antigens (Ag)

	ANTIGENS, also called IMMUNOGENS, cause the body to produce a 
highly specific immune response in the form of ANTIBODIES (Ab) and/or 
SPECIALLY SENSITIZED CELLS. The vast majority of antigens are proteins, 
large polysaccharides, or combinations of either (nucleoproteins, 
lipoproteins, glycoproteins). Because they represent macromolecules that 
are normally not present in the body they are called "NON-SELF"
molecules. Milk proteins, bee venom, hemoglobin molecules, bacterial toxins 
and various bacterial and viral components may serve as antigens.
	Only a portion of the antigen is attached to the antibody. The 
portion of the antigen (Ag) where the Ag and Ab interact is called 
the ANTIGENIC DETERMINANT SITE. A large antigen may have several 
antigenic determinant sites. These antigenic determinants are like 
fingerprints to which the various parts of the immune system respond after 
they have been identified. The nature of this interaction depends upon the 
size and shape of the antigenic determinant site in relation to the chemical
structure of the antibody. Certain low molecular weight substances, which 
by themselves are initially NOT antigenic, can become antigenic when they 
become attached to carrier molecules (such as proteins or polysaccharides). 
These low molecular weight substances are called HAPTENS. Once an antibody 
has been formed against a hapten-carrier complex, the hapten alone will 
stimulate further Ab production. The beta-lactam backbone of the penicillin 
molecule may become attached to serum proteins in some persons. Antibodies 
are formed against this new large complex and subsequently penicillin and 
all other molecules with the beta-lactam backbone become recognizable Ag's.

	Normally a person's own chemical substances do not stimulate an 
immune response because they are interpreted as "SELF" molecules.
When a person's tolerance of SELF molecules breaks down, an immune response 
may occur. This may result in damage or destruction of various cells or 
tissues and may be called an AUTO-IMMUNE RESPONSE.

Antibodies (Ab)

	ANTIBODIES are proteins that are produced by B-lymphocytes in 
response to the presence of an antigen. The Ab is capable of combining 
specifically with that Ag. The Ab has more than one combining site. 
Antibodies belong to a group of proteins called GLOBULINS. Because they 
are involved in the immune response they have a general name of 
IMMUNOGLOBULINS (Ig). Immunoglobulins are found in the blood serum, mainly 
in the GAMMA fraction, and in other bodily secretions. 
	Antibodies are often identified in many different ways. We have 
seen that because of their protein organizational level they are globulins. 
Being involved in immune responses these protein antibodies are called 
immunoglobulins. Antibodies are sometimes identified functionally. For 
example Ab's that neutralize toxins are called ANTITOXINS; those that cause 
precipitation reactions are called PRECIPITINS. The structure, size, and 
location of Ab's is sometimes used to identify them. At the present time, 
five classes of antibody (immunoglobulin) are recognized based on 
differences in part of their structure. These classes of Ig,s are designated 
IgM, IgG, IgA, IgE, and IgD. 


Classes of Immunoglobulins

Ig-M	the very first time that we are exposed or challenged
	by an new antigen (one the body has never recognized before)
	this very large antibody molecule is formed; its large size 
	prevents it from leaving the general circulation; the anti-
	gen becomes identified by being phagocytized by macrophages
	which are then contacted by B-cells; these B-cells begin to 
	produce the IgM and come in contact with T-cells which then
	become sensitized and stimulate the B-cells to change into
	antibody factories known as plasma cells; this sequence 
	begins a production of a second class of Ig's known as IgG's
	IgM antibodies are involved in ABO blood typing reactions, 
	Complement Fixation reactions and enhanced phagocytosis

Ig-G		for long-term immunity to develop IgG antibodies must
	be produced; 80-85% of all circulating Ab's are IgG; they 	
        are found in the blood, the lymph, and the intestinal fluids
	as they are capable of crossing the walls of blood vessels
	and entering the tissue fluids; they protect against circu-
	lating bacteria and viruses; they can neutralize bacterial 
	toxins and aid phagocytic cells; they also participate in 
	Complement Fixation reactions and are the type of Ab found
	in the first mother's milk and passed on to new babies; this
	group can be subdivided into at least four subclasses

Ig-A		about 15% of all Ab's are of this type; they are found	
	in the serum and in secretions of the gastrointestinal tract
	the respiratory system, and other mucous membranes; their 
	main function is to prevent pathogens from attaching to the
	surfaces of mucous membranes; secretory IgA in colostrum 
	provides passive immune protection to the gut of the newborn
	against microbes that might cause gastroenteritis; in resp-
	iratory mucous IgA may neutralize allergens

Ig-E		making up less than 1% of the total serum Ig's this 
	group is responsible for some of the most severe immunologic
	reactions called hypersensitivity or allergic reactions; 	
	this Ig binds to tissue basophils (called mast cells) and in
	combination with the antigen causes the basophil to degranu-
	late (break open its lysosomal granules) and release 	
	histamine, heparin, leukotrienes and serotonin which are 
	responsible for most of the reactions associated with 
	allergic or hypersensitivity reactions

Ig-D		little is known about this type of Ig which makes up
	about 1% of all AB's; it may be involved in reactions 
	against cow's milk; in fetal lymphocytes it may determine
	which cells should be destroyed so that they do not cause
	autoimmune reactions

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Email: demmelut@ps.fmcc.suny.edu