Answer:
characterized by presence or absence of antigens
the blood types are A, B, O, AB
Explanation:
There are two antigens and two antibodies that are mostly responsible for the ABO types.  The specific combination of these four components determines an individual's type in most cases. Erythrocytes and serum were related to the presence of antigens on these erythrocytes and antibodies in the serum. these antigens  are A and B, and depending upon which antigen the erythrocytes express, blood either belonged to blood group A or blood group B. A third blood group contained erythrocytes that reacted as if they lacked the properties of A and B, and this group was later called "O" blood group. The fourth blood group AB, was added to the ABO blood group system. These erythrocytes expressed both A and B antigens. 
Blood group   Antigen present on RBC    Antibodies in serum    Genotype(s)
       A                        antigen A                             anti-B                     AA or AO
       B                        antigen B                             anti-A                     BB or BO
       AB              both A and B antigen                 none                         AB
       O                           none                           anti-A and anti-B           OO
 
        
             
        
        
        
decreased ability to survive changing conditions
Hope this helps
 
        
             
        
        
        
A nerve is actually a long threadlike
bundle of dendrites that conduct electrical impulses.
<span>Dendrite word derived from the Greek
word ‘dendron’ which means tree. They carry messages in the form of electrical
impulses to cell body, there are also wire like nerves called axon. Sensory
nerves and motor nerves are common types of nerves.</span>
 
        
             
        
        
        
4 i guess........................