This chapter presents a glimpse on repeated infections which occurs due to immunodeficiency. Primary immunodeficiency (PID) is relatively rare in adults and generally manifest in children, comprises of a group of clinical diseases with a variety of underlying genetic and molecular mechanisms. PID should be considered in patients with frequent, multiple or prolonged infections caused by certain pathogens such as Streptococcus pneumoniae, Neisseria spp, and Haemophilus influenzae. IgA deficiency is the most common primary immunodeficiency. IgA is the major Ig in external secretion and blocks the binding of antigens to cell receptors. Common variable immunodeficiency (CVID) occurs both in adults and children. CVID is caused by impaired B cell differentiations and defective Ig production. Patients with CVID frequently develop recurrent sinopulmonary and ear infections, autoimmune disorders, malabsorption, and lymphoma. Complement deficiencies represent the least common of all primary immune deficiencies. They are inherited as autosomal recessive disorders and can be categorized into two major groups i.e. deficiencies affecting the early components and deficiencies affecting the terminal components.