Involvement of central nervous system (CNS) leukemia is defined by the presence of lymphoblast in the cerebrospinal fluid. It is found in 1.2–10 % of children with newly diagnosed ALL. Leukemic blasts entering the CNS by hematogenous spread. CNS leukemia is more common in mature B cell, T-ALL and children with high WBC. Signs of CNS involvement:

Leukemic involvement in the lungs and heart is rare. The manifestations could be: pericardial leukemic effusion and mediastinal mass, especially in T-ALL. Late cardiomyopathy is found after extensive treatment with anthracyclines [48].

Mediastinal mass (thymus enlargement) due to leukemic infiltration, may cause life-threatening. Especially in T-ALL: superior vena cava syndrome.

Bleeding (retinal bleeding) caused by high white blood cell count and/or thrombocytopenia [28].

Involvement of musculoskeletal is characterized by severe pain, especially in lower extremities and sometimes unable/refusal to walk. This symptom occurred in 20–30 % of children with ALL. It may result of infiltration of leukemic cells to the bone or expansion of marrow cavity by leukemic cells. It may also appear swelling and tenderness due to leukemic infiltration [49, 50].

Involvement of testicles is present mostly in boys. Testicular involvement is diagnosed if leukemic blasts found by testicular biopsy. It occurred only in boys with WBC >25,000/mm³, T-cell ALL, moderate to severe hepatosplenomegaly, lymphadenopathy and thrombocytopenia (<30,000/mm³). In girls, ovarian involvement occurs very rare [51–53].

The commonest manifestation of leukemia in GIT is bleeding, as reflected by occult blood in the stool. GIT bleeding might also be caused by thrombocytopenia, disseminated intravascular coagulation (DIC) or infection. Neutropenic typhlitis or necrotizing enterocolitis diagnosed if right lower quadrant pain with tenderness, abdominal tension, vomiting and sepsis are found [46].

Leukemia must be suspected when the bone marrow contains more than 5 % blasts. The hallmark of the diagnosis of acute leukemia is the blast cell (more than 25 %), a relatively undifferentiated cell with diffusely distributed nuclear chromatin, one or more nucleoli, and basophilic cytoplasm. Special bone marrow studies, will help in detailed classification, include the following: Cytochemistry, Immunophenotyping, Cytogenetic and DNA content [55]. Bone marrow smear stained with other May-Grünwald-Giemsa or Wright-Giemsa, and should be examined under a light microscope. It is important to examine the morphology to distinguish lymphoblast and myeloblast. Acute leukemia can be classified based on morphologic characteristics into lymphoblastic leukemia and myeloblastic leukemia (Table 43.3a) [44].

Cytochemical features is needed to sharpen the diagnosis [28]. Cytochemistry for myeloperoxidase and non-specific esterase should be done to exclude acute myeloid leukemia [62, 63]. To support diagnosis of ALL, cytochemistry such as periodic acid shift (PAS), peroxidase and Sudan-Black staining are recommended (Table 43.3b).

The French-American-British (FAB) Working Group Classification of ALL is based on morphologic and cytochemical features. Peripheral blood and bone marrow smears are stained (May-Grünwald-Giemsa method) and analyzed using light microscopy. Leukemic cells are characterized by a lack of differentiation, by a nucleus with diffuse chromatin structure, with one or more nucleoli, and by basophilic cytoplasm. This morphologic classification system categorizes lymphoblastic leukemias into three subtypes: L1, L2 or L3 (Table 43.4) [44].

Immunophenotyping of abnormal hematological cells using flow cytometry studies on peripheral blood or bone marrow samples improves both accuracy and reproducibility of classification of acute leukemias [64–69]. It is very useful for the diagnosis, classification, cost-effective treatment and prognostic evaluation in patients with hematological malignancies [64, 70–72]. Usually, ALL is classified into T-lineage, B-lineage, and B-cell (Burkitt’s) phenotypes. The World Health Organization (WHO) classification (Table43.5) divides ALL into two main groups only, i.e., B-lineage and T-lineage ALL, without further categorization [73–76].

Classification of acute leukemia (B- or T-lineage ALL/AML) is based on reactivity patterns obtained with a panel of lineage-associated antibodies [77–79]. Immunophenotyping is also essential for distinguishing between ALL and AML; errors in differentiating between these two types of acute leukemias can occur in up to 10 % of cases [80–83]. Essential monoclonal antibodies for detecting acute leukemia are presented in Table 43.6.

The B-lineage phenotype ALL, positive for the following: B cell markers CD19, CD20, CD22, TdT, cytoplasmic CD79a, CD34 and CD10. It has been sub-classified according to maturation stage into: early pre B (pro-B), pre-B, transitional (or late) pre-B and (mature) B-ALL [64, 84]. In different regions, various incidences of B-lineage ALL have been reported. Burkitt’s leukemia displays an immunophenotype consisting of mature B cells [78].