The scope of this chapter was to present the main aspects of the immune response. They are summarized as follows : (i) Specificity ensures the immune response to be directed against the targeted microbe/antigen ; (ii) Diversity enables the spreading of the immune response towards a large variety of antigens ; (iii) Memory increase the ability to fight relapses associated to the same antigen ; (iv) Clonal expansion provides a number of antigen-specific lymphocytes ; (v) Specialization generates clones with a more defined specificity for their antigen.