**1.8 Primary immune response**

It is the first exposure to the Ag resulting of forming specific Abs and memory B-cells for HMI or T-cells and memory T-cells for CMI, the phases are, **Figure 2** [1–5]:

	- No Ab level increase (Steady titer).
	- Recognizing Ag as foreign after processing the Ag inside APC.
	- Cellular proliferation and differentiation.
	- Duration of this phase (period) is variable depending on many factors (Ag immunogenicity, Ag dose, Ag solubility, Ag route of immunization or exposure).

Note: during early primary response, IgM class antibodies is predominant and first rise than IgG appears later [2, 5].

**Figure 2.** *Primary and secondary immune response.*

## **1.9 Secondary immune response**

It is the second exposure to the same immunogen that induced the first immune response (after booster dose of vaccination) may be after weeks, months, or even years later, includes [1–5]:


Negative phase: occur between primary and secondary Immune response when immunogen second dose is small and/or there is pre-existing antibodies from the first immune response (primary), then immunogen will be all consumed in Ag-Ab complex formation and phagocytosed then removed with no induction to secondary immune response [5].
